Generation of chroma components using cross-component adaptive loop filters

ABSTRACT

A method for video encoding includes determining a filter shape of a cross-component filter applied to a chroma coding block (CB), generating a first intermediate CB by applying a loop filter to the chroma CB, and generating a second intermediate CB by applying, to a corresponding luma CB, the cross-component filter applied to the chroma CB and having the determined filter shape. The method further includes determining a filtered chroma CB based on the first intermediate CB and the second intermediate CB by combining the loop filtered chroma CB with the cross-component filtered luma CB, and generating coded information of the chroma CB in a coded video bitstream. Determining the filter shape includes determining the filter shape of the cross-component filter based on the number of the filter coefficients and based on at least one of (i) the chroma subsampling format or (ii) the chroma sample type.

INCORPORATION BY REFERENCE

The application is a continuation of U.S. Ser. No. 17/010,403 filed Sep.2, 2020, which claims the benefit of priority to U.S. ProvisionalApplication No. 62/901,118, “Of Cross-Component Adaptive Loop Filter”filed on Sep. 16, 2019. The disclosures of the prior applications arehereby incorporated by reference in their entirety.

TECHNICAL FIELD

The present disclosure describes embodiments generally related to videocoding.

BACKGROUND

The background description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent the work is described in thisbackground section, as well as aspects of the description that may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentdisclosure.

Video coding and decoding can be performed using inter-pictureprediction with motion compensation. Uncompressed digital video caninclude a series of pictures, each picture having a spatial dimensionof, for example, 1920×1080 luminance samples and associated chrominancesamples. The series of pictures can have a fixed or variable picturerate (informally also known as frame rate), of, for example 60 picturesper second or 60 Hz. Uncompressed video has specific bitraterequirements. For example, 1080p60 4:2:0 video at 8 bit per sample(1920×1080 luminance sample resolution at 60 Hz frame rate) requiresclose to 1.5 Gbit/s bandwidth. An hour of such video requires more than600 GBytes of storage space.

One purpose of video coding and decoding can be the reduction ofredundancy in the input video signal, through compression. Compressioncan help reduce the aforementioned bandwidth and/or storage spacerequirements, in some cases by two orders of magnitude or more. Bothlossless compression and lossy compression, as well as a combinationthereof can be employed. Lossless compression refers to techniques wherean exact copy of the original signal can be reconstructed from thecompressed original signal. When using lossy compression, thereconstructed signal may not be identical to the original signal, butthe distortion between original and reconstructed signals is smallenough to make the reconstructed signal useful for the intendedapplication. In the case of video, lossy compression is widely employed.The amount of distortion tolerated depends on the application; forexample, users of certain consumer streaming applications may toleratehigher distortion than users of television distribution applications.The compression ratio achievable can reflect that: higherallowable/tolerable distortion can yield higher compression ratios.

A video encoder and decoder can utilize techniques from several broadcategories, including, for example, motion compensation, transform,quantization, and entropy coding.

Video codec technologies can include techniques known as intra coding.In intra coding, sample values are represented without reference tosamples or other data from previously reconstructed reference pictures.In some video codecs, the picture is spatially subdivided into blocks ofsamples. When all blocks of samples are coded in intra mode, thatpicture can be an intra picture. Intra pictures and their derivationssuch as independent decoder refresh pictures, can be used to reset thedecoder state and can, therefore, be used as the first picture in acoded video bitstream and a video session, or as a still image. Thesamples of an intra block can be exposed to a transform, and thetransform coefficients can be quantized before entropy coding. Intraprediction can be a technique that minimizes sample values in thepre-transform domain. In some cases, the smaller the DC value after atransform is, and the smaller the AC coefficients are, the fewer thebits that are required at a given quantization step size to representthe block after entropy coding.

Traditional intra coding such as known from, for example MPEG-2generation coding technologies, does not use intra prediction. However,some newer video compression technologies include techniques thatattempt, from, for example, surrounding sample data and/or metadataobtained during the encoding/decoding of spatially neighboring, andpreceding in decoding order, blocks of data. Such techniques arehenceforth called “intra prediction” techniques. Note that in at leastsome cases, intra prediction is using reference data only from thecurrent picture under reconstruction and not from reference pictures.

There can be many different forms of intra prediction. When more thanone of such techniques can be used in a given video coding technology,the technique in use can be coded in an intra prediction mode. Incertain cases, modes can have submodes and/or parameters, and those canbe coded individually or included in the mode codeword. Which codewordto use for a given mode/submode/parameter combination can have an impactin the coding efficiency gain through intra prediction, and so can theentropy coding technology used to translate the codewords into abitstream.

A certain mode of intra prediction was introduced with H.264, refined inH.265, and further refined in newer coding technologies such as jointexploration model (JEM), versatile video coding (VVC), and benchmark set(BMS). A predictor block can be formed using neighboring sample valuesbelonging to already available samples. Sample values of neighboringsamples are copied into the predictor block according to a direction. Areference to the direction in use can be coded in the bitstream or mayitself be predicted.

Referring to FIG. 1A, depicted in the lower right is a subset of ninepredictor directions known from H.265's 33 possible predictor directions(corresponding to the 33 angular modes of the 35 intra modes). The pointwhere the arrows converge (101) represents the sample being predicted.The arrows represent the direction from which the sample is beingpredicted. For example, arrow (102) indicates that sample (101) ispredicted from a sample or samples to the upper right, at a 45 degreeangle from the horizontal. Similarly, arrow (103) indicates that sample(101) is predicted from a sample or samples to the lower left of sample(101), in a 22.5 degree angle from the horizontal.

Still referring to FIG. 1A, on the top left there is depicted a squareblock (104) of 4×4 samples (indicated by a dashed, boldface line). Thesquare block (104) includes 16 samples, each labelled with an “S”, itsposition in the Y dimension (e.g., row index) and its position in the Xdimension (e.g., column index). For example, sample S21 is the secondsample in the Y dimension (from the top) and the first (from the left)sample in the X dimension. Similarly, sample S44 is the fourth sample inblock (104) in both the Y and X dimensions. As the block is 4×4 samplesin size, S44 is at the bottom right. Further shown are reference samplesthat follow a similar numbering scheme. A reference sample is labelledwith an R, its Y position (e.g., row index) and X position (columnindex) relative to block (104). In both H.264 and H.265, predictionsamples neighbor the block under reconstruction; therefore no negativevalues need to be used.

Intra picture prediction can work by copying reference sample valuesfrom the neighboring samples as appropriated by the signaled predictiondirection. For example, assume the coded video bitstream includessignaling that, for this block, indicates a prediction directionconsistent with arrow (102)—that is, samples are predicted from aprediction sample or samples to the upper right, at a 45 degree anglefrom the horizontal. In that case, samples S41, S32, S23, and S14 arepredicted from the same reference sample R05. Sample S44 is thenpredicted from reference sample R08.

In certain cases, the values of multiple reference samples may becombined, for example through interpolation, in order to calculate areference sample; especially when the directions are not evenlydivisible by 45 degrees.

The number of possible directions has increased as video codingtechnology has developed. In H.264 (year 2003), nine different directioncould be represented. That increased to 33 in H.265 (year 2013), andJEM/VVC/BMS, at the time of disclosure, can support up to 65 directions.Experiments have been conducted to identify the most likely directions,and certain techniques in the entropy coding are used to represent thoselikely directions in a small number of bits, accepting a certain penaltyfor less likely directions. Further, the directions themselves cansometimes be predicted from neighboring directions used in neighboring,already decoded, blocks.

FIG. 1B shows a schematic (180) that depicts 65 intra predictiondirections according to JEM to illustrate the increasing number ofprediction directions over time.

The mapping of intra prediction directions bits in the coded videobitstream that represent the direction can be different from videocoding technology to video coding technology; and can range, forexample, from simple direct mappings of prediction direction to intraprediction mode, to codewords, to complex adaptive schemes involvingmost probable modes, and similar techniques. In all cases, however,there can be certain directions that are statistically less likely tooccur in video content than certain other directions. As the goal ofvideo compression is the reduction of redundancy, those less likelydirections will, in a well working video coding technology, berepresented by a larger number of bits than more likely directions.

Motion compensation can be a lossy compression technique and can relateto techniques where a block of sample data from a previouslyreconstructed picture or part thereof (reference picture), after beingspatially shifted in a direction indicated by a motion vector (MVhenceforth), is used for the prediction of a newly reconstructed pictureor picture part. In some cases, the reference picture can be the same asthe picture currently under reconstruction. MVs can have two dimensionsX and Y, or three dimensions, the third being an indication of thereference picture in use (the latter, indirectly, can be a timedimension).

In some video compression techniques, an MV applicable to a certain areaof sample data can be predicted from other MVs, for example from thoserelated to another area of sample data spatially adjacent to the areaunder reconstruction, and preceding that MV in decoding order. Doing socan substantially reduce the amount of data required for coding the MV,thereby removing redundancy and increasing compression. MV predictioncan work effectively, for example, because when coding an input videosignal derived from a camera (known as natural video) there is astatistical likelihood that areas larger than the area to which a singleMV is applicable move in a similar direction and, therefore, can in somecases be predicted using a similar motion vector derived from MVs ofneighboring area. That results in the MV found for a given area to besimilar or the same as the MV predicted from the surrounding MVs, andthat in turn can be represented, after entropy coding, in a smallernumber of bits than what would be used if coding the MV directly. Insome cases, MV prediction can be an example of lossless compression of asignal (namely: the MVs) derived from the original signal (namely: thesample stream). In other cases, MV prediction itself can be lossy, forexample because of rounding errors when calculating a predictor fromseveral surrounding MVs.

Various MV prediction mechanisms are described in H.265/HEVC (ITU-T Rec.H.265, “High Efficiency Video Coding”, December 2016). Out of the manyMV prediction mechanisms that H.265 offers, described here is atechnique henceforth referred to as “spatial merge”.

Referring to FIG. 2, a current block (201) comprises samples that havebeen found by the encoder during the motion search process to bepredictable from a previous block of the same size that has beenspatially shifted. Instead of coding that MV directly, the MV can bederived from metadata associated with one or more reference pictures,for example from the most recent (in decoding order) reference picture,using the MV associated with either one of five surrounding samples,denoted A0, A1, and B0, B1, B2 (202 through 206, respectively). InH.265, the MV prediction can use predictors from the same referencepicture that the neighboring block is using.

SUMMARY

Aspects of the disclosure provide methods and apparatuses for videoencoding/decoding. In some examples, an apparatus for video decodingincludes processing circuitry. The processing circuitry can decode codedinformation of a chroma coding block (CB) from a coded video bitstream.The coded information can indicate that a cross-component filter isapplied to the chroma CB. The coded information can further indicate achroma subsampling format and a chroma sample type that indicates arelative position of a chroma sample with respect to at least one lumasample in the a corresponding luma CB. The processing circuitry candetermine a filter shape of the cross-component filter based on at leastone of the chroma subsampling format and the chroma sample type. Theprocessing circuitry can generate a first intermediate CB by applying aloop filter to the chroma CB. The processing circuitry can generate asecond intermediate CB by applying the cross-component filter having thedetermined filter shape to the corresponding luma CB. The processingcircuitry can determine a filtered chroma CB based on the firstintermediate CB and the second intermediate CB.

In an example, the chroma sample type is signaled in the coded videobitstream.

In an example, a number of filter coefficients of the cross-componentfilter is signaled in the coded video bitstream. The processingcircuitry can determine the filter shape of the cross-component filterbased on the number of filter coefficients and the at least one of thechroma subsampling format and the chroma sample type.

In an example, the chroma subsampling format is 4:2:0. The at least oneluma sample includes four luma samples that are a top-left sample, atop-right sample, a bottom-left sample, and a bottom-right sample. Thechroma sample type is one of six chroma sample types 0-5 indicating sixrelative positions 0-5, respectively, and the six relative positions 0-5of the chroma sample correspond to a left-center position between thetop-left and the bottom-left samples, a center position of the four lumasamples, a top left position co-located with the top-left sample, atop-center position between the top-left and the top-right samples, abottom left position co-located with the bottom-left sample, and abottom-center position between the bottom-left and the bottom-rightsamples, respectively. The processing circuitry can determine the filtershape of the cross-component filter based on the chroma sample type. Inan example, the coded video bitstream includes a cross-component linearmodel (CCLM) flag indicating that the chroma sample type is 0 or 2.

In an example, the cross-component filter is a cross-component adaptiveloop filter (CC-ALF) and the loop filter is an adaptive loop filter(ALF).

In an embodiment, a range of filter coefficients of the cross-componentfilter is less than or equal to K bits and K is a positive integer. Inan example, the filter coefficients of the cross-component filter arecoded using fixed-length coding. In an example, the processing circuitrycan shift luma sample values of the corresponding luma CB to have adynamic range of 8 bits based on the dynamic range of the luma samplevalues being larger than 8 bits where K is 8 bits. The processingcircuitry can apply the cross-component filter having the determinedfilter shape to the shifted luma sample values.

In some examples, an apparatus for video decoding includes processingcircuitry. The processing circuitry can decode coded information of achroma CB from a coded video bitstream. The coded information canindicate that a cross-component filter is applied to the chroma CB basedon a corresponding luma CB. The processing circuitry can generate adown-sampled luma CB by applying a down-sampling filter to thecorresponding luma CB where a chroma horizontal subsampling factor and achroma vertical subsampling factor between the chroma CB and thedown-sampled luma CB is one. The processing circuitry can generate afirst intermediate CB by applying a loop filter to the chroma CB. Theprocessing circuitry can generate a second intermediate CB by applyingthe cross-component filter to the down-sampled luma CB where a filtershape of the cross-component filter is independent of a chromasubsampling format and a chroma sample type of the chroma CB. The chromasample type can indicate a relative position of a chroma sample withrespect to at least one luma sample in the corresponding luma CB. Theprocessing circuitry can determine a filtered chroma CB based on thefirst intermediate CB and the second intermediate CB. In an example, thedown-sampling filter corresponds to a filter applied to co-located lumasamples in a CCLM mode. In an example, the down-sampling filter is a{1,2,1;1,2,1}/8 filter and the chroma subsampling format is 4:2:0. In anexample, the filter shape of the cross-component filter is one of a 7×7diamond shape, a 7×7 square shape, a 5×5 diamond shape, a 5×5 squareshape, a 3×3 diamond shape, and a 3×3 square shape.

Aspects of the disclosure also provide a non-transitorycomputer-readable medium storing instructions which when executed by acomputer for video decoding cause the computer to perform any of themethods for video decoding.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features, the nature, and various advantages of the disclosedsubject matter will be more apparent from the following detaileddescription and the accompanying drawings in which:

FIG. 1A is a schematic illustration of an exemplary subset of intraprediction modes.

FIG. 1B is an illustration of exemplary intra prediction directions.

FIG. 2 is a schematic illustration of a current block and itssurrounding spatial merge candidates in one example.

FIG. 3 is a schematic illustration of a simplified block diagram of acommunication system (300) in accordance with an embodiment.

FIG. 4 is a schematic illustration of a simplified block diagram of acommunication system (400) in accordance with an embodiment.

FIG. 5 is a schematic illustration of a simplified block diagram of adecoder in accordance with an embodiment.

FIG. 6 is a schematic illustration of a simplified block diagram of anencoder in accordance with an embodiment.

FIG. 7 shows a block diagram of an encoder in accordance with anotherembodiment.

FIG. 8 shows a block diagram of a decoder in accordance with anotherembodiment.

FIG. 9 shows examples of filter shapes according to embodiments of thedisclosure.

FIGS. 10A-10D show examples of subsampled positions used for calculatinggradients according to embodiments of the disclosure.

FIGS. 11A-11B show examples of a virtual boundary filtering processaccording to embodiments of the disclosure.

FIGS. 12A-12F show examples of symmetric padding operations at virtualboundaries according to embodiments of the disclosure.

FIG. 13 shows an exemplary functional diagram for generating luma andchroma components according to an embodiment of the disclosure.

FIG. 14 shows an example of a filter 1400 according to an embodiment ofthe disclosure.

FIGS. 15A-15B show exemplary locations of chroma samples relative toluma samples according to embodiments of the disclosure.

FIG. 16 shows examples of filter shapes (1601)-(1603) of respectivecross-component adaptive loop filters (CC-ALFs) according to embodimentsof the disclosure.

FIG. 17 shows a flow chart outlining a process (1700) according to anembodiment of the disclosure.

FIG. 18 is a schematic illustration of a computer system in accordancewith an embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 3 illustrates a simplified block diagram of a communication system(300) according to an embodiment of the present disclosure. Thecommunication system (300) includes a plurality of terminal devices thatcan communicate with each other, via, for example, a network (350). Forexample, the communication system (300) includes a first pair ofterminal devices (310) and (320) interconnected via the network (350).In the FIG. 3 example, the first pair of terminal devices (310) and(320) performs unidirectional transmission of data. For example, theterminal device (310) may code video data (e.g., a stream of videopictures that are captured by the terminal device (310)) fortransmission to the other terminal device (320) via the network (350).The encoded video data can be transmitted in the form of one or morecoded video bitstreams. The terminal device (320) may receive the codedvideo data from the network (350), decode the coded video data torecover the video pictures and display video pictures according to therecovered video data. Unidirectional data transmission may be common inmedia serving applications and the like.

In another example, the communication system (300) includes a secondpair of terminal devices (330) and (340) that performs bidirectionaltransmission of coded video data that may occur, for example, duringvideoconferencing. For bidirectional transmission of data, in anexample, each terminal device of the terminal devices (330) and (340)may code video data (e.g., a stream of video pictures that are capturedby the terminal device) for transmission to the other terminal device ofthe terminal devices (330) and (340) via the network (350). Eachterminal device of the terminal devices (330) and (340) also may receivethe coded video data transmitted by the other terminal device of theterminal devices (330) and (340), and may decode the coded video data torecover the video pictures and may display video pictures at anaccessible display device according to the recovered video data.

In the FIG. 3 example, the terminal devices (310), (320), (330) and(340) may be illustrated as servers, personal computers and smart phonesbut the principles of the present disclosure may be not so limited.Embodiments of the present disclosure find application with laptopcomputers, tablet computers, media players and/or dedicated videoconferencing equipment. The network (350) represents any number ofnetworks that convey coded video data among the terminal devices (310),(320), (330) and (340), including for example wireline (wired) and/orwireless communication networks. The communication network (350) mayexchange data in circuit-switched and/or packet-switched channels.Representative networks include telecommunications networks, local areanetworks, wide area networks and/or the Internet. For the purposes ofthe present discussion, the architecture and topology of the network(350) may be immaterial to the operation of the present disclosureunless explained herein below.

FIG. 4 illustrates, as an example for an application for the disclosedsubject matter, the placement of a video encoder and a video decoder ina streaming environment. The disclosed subject matter can be equallyapplicable to other video enabled applications, including, for example,video conferencing, digital TV, storing of compressed video on digitalmedia including CD, DVD, memory stick and the like, and so on.

A streaming system may include a capture subsystem (413), that caninclude a video source (401), for example a digital camera, creating forexample a stream of video pictures (402) that are uncompressed. In anexample, the stream of video pictures (402) includes samples that aretaken by the digital camera. The stream of video pictures (402),depicted as a bold line to emphasize a high data volume when compared toencoded video data (404) (or coded video bitstreams), can be processedby an electronic device (420) that includes a video encoder (403)coupled to the video source (401). The video encoder (403) can includehardware, software, or a combination thereof to enable or implementaspects of the disclosed subject matter as described in more detailbelow. The encoded video data (404) (or encoded video bitstream (404)),depicted as a thin line to emphasize the lower data volume when comparedto the stream of video pictures (402), can be stored on a streamingserver (405) for future use. One or more streaming client subsystems,such as client subsystems (406) and (408) in FIG. 4 can access thestreaming server (405) to retrieve copies (407) and (409) of the encodedvideo data (404). A client subsystem (406) can include a video decoder(410), for example, in an electronic device (430). The video decoder(410) decodes the incoming copy (407) of the encoded video data andcreates an outgoing stream of video pictures (411) that can be renderedon a display (412) (e.g., display screen) or other rendering device (notdepicted). In some streaming systems, the encoded video data (404),(407), and (409) (e.g., video bitstreams) can be encoded according tocertain video coding/compression standards. Examples of those standardsinclude ITU-T Recommendation H.265. In an example, a video codingstandard under development is informally known as Versatile Video Coding(VVC). The disclosed subject matter may be used in the context of VVC.

It is noted that the electronic devices (420) and (430) can includeother components (not shown). For example, the electronic device (420)can include a video decoder (not shown) and the electronic device (430)can include a video encoder (not shown) as well.

FIG. 5 shows a block diagram of a video decoder (510) according to anembodiment of the present disclosure. The video decoder (510) can beincluded in an electronic device (530). The electronic device (530) caninclude a receiver (531) (e.g., receiving circuitry). The video decoder(510) can be used in the place of the video decoder (410) in the FIG. 4example.

The receiver (531) may receive one or more coded video sequences to bedecoded by the video decoder (510); in the same or another embodiment,one coded video sequence at a time, where the decoding of each codedvideo sequence is independent from other coded video sequences. Thecoded video sequence may be received from a channel (501), which may bea hardware/software link to a storage device which stores the encodedvideo data. The receiver (531) may receive the encoded video data withother data, for example, coded audio data and/or ancillary data streams,that may be forwarded to their respective using entities (not depicted).The receiver (531) may separate the coded video sequence from the otherdata. To combat network jitter, a buffer memory (515) may be coupled inbetween the receiver (531) and an entropy decoder/parser (520) (“parser(520)” henceforth). In certain applications, the buffer memory (515) ispart of the video decoder (510). In others, it can be outside of thevideo decoder (510) (not depicted). In still others, there can be abuffer memory (not depicted) outside of the video decoder (510), forexample to combat network jitter, and in addition another buffer memory(515) inside the video decoder (510), for example to handle playouttiming. When the receiver (531) is receiving data from a store/forwarddevice of sufficient bandwidth and controllability, or from anisosynchronous network, the buffer memory (515) may not be needed, orcan be small. For use on best effort packet networks such as theInternet, the buffer memory (515) may be required, can be comparativelylarge and can be advantageously of adaptive size, and may at leastpartially be implemented in an operating system or similar elements (notdepicted) outside of the video decoder (510).

The video decoder (510) may include the parser (520) to reconstructsymbols (521) from the coded video sequence. Categories of those symbolsinclude information used to manage operation of the video decoder (510),and potentially information to control a rendering device such as arender device (512) (e.g., a display screen) that is not an integralpart of the electronic device (530) but can be coupled to the electronicdevice (530), as was shown in FIG. 5. The control information for therendering device(s) may be in the form of Supplemental EnhancementInformation (SEI messages) or Video Usability Information (VUI)parameter set fragments (not depicted). The parser (520) mayparse/entropy-decode the coded video sequence that is received. Thecoding of the coded video sequence can be in accordance with a videocoding technology or standard, and can follow various principles,including variable length coding, Huffman coding, arithmetic coding withor without context sensitivity, and so forth. The parser (520) mayextract from the coded video sequence, a set of subgroup parameters forat least one of the subgroups of pixels in the video decoder, based uponat least one parameter corresponding to the group. Subgroups can includeGroups of Pictures (GOPs), pictures, tiles, slices, macroblocks, CodingUnits (CUs), blocks, Transform Units (TUs), Prediction Units (PUs) andso forth. The parser (520) may also extract from the coded videosequence information such as transform coefficients, quantizer parametervalues, motion vectors, and so forth.

The parser (520) may perform an entropy decoding/parsing operation onthe video sequence received from the buffer memory (515), so as tocreate symbols (521).

Reconstruction of the symbols (521) can involve multiple different unitsdepending on the type of the coded video picture or parts thereof (suchas: inter and intra picture, inter and intra block), and other factors.Which units are involved, and how, can be controlled by the subgroupcontrol information that was parsed from the coded video sequence by theparser (520). The flow of such subgroup control information between theparser (520) and the multiple units below is not depicted for clarity.

Beyond the functional blocks already mentioned, the video decoder (510)can be conceptually subdivided into a number of functional units asdescribed below. In a practical implementation operating undercommercial constraints, many of these units interact closely with eachother and can, at least partly, be integrated into each other. However,for the purpose of describing the disclosed subject matter, theconceptual subdivision into the functional units below is appropriate.

A first unit is the scaler/inverse transform unit (551). Thescaler/inverse transform unit (551) receives a quantized transformcoefficient as well as control information, including which transform touse, block size, quantization factor, quantization scaling matrices,etc. as symbol(s) (521) from the parser (520). The scaler/inversetransform unit (551) can output blocks comprising sample values, thatcan be input into aggregator (555).

In some cases, the output samples of the scaler/inverse transform (551)can pertain to an intra coded block; that is: a block that is not usingpredictive information from previously reconstructed pictures, but canuse predictive information from previously reconstructed parts of thecurrent picture. Such predictive information can be provided by an intrapicture prediction unit (552). In some cases, the intra pictureprediction unit (552) generates a block of the same size and shape ofthe block under reconstruction, using surrounding already reconstructedinformation fetched from the current picture buffer (558). The currentpicture buffer (558) buffers, for example, partly reconstructed currentpicture and/or fully reconstructed current picture. The aggregator(555), in some cases, adds, on a per sample basis, the predictioninformation the intra prediction unit (552) has generated to the outputsample information as provided by the scaler/inverse transform unit(551).

In other cases, the output samples of the scaler/inverse transform unit(551) can pertain to an inter coded, and potentially motion compensatedblock. In such a case, a motion compensation prediction unit (553) canaccess reference picture memory (557) to fetch samples used forprediction. After motion compensating the fetched samples in accordancewith the symbols (521) pertaining to the block, these samples can beadded by the aggregator (555) to the output of the scaler/inversetransform unit (551) (in this case called the residual samples orresidual signal) so as to generate output sample information. Theaddresses within the reference picture memory (557) from where themotion compensation prediction unit (553) fetches prediction samples canbe controlled by motion vectors, available to the motion compensationprediction unit (553) in the form of symbols (521) that can have, forexample X, Y, and reference picture components. Motion compensation alsocan include interpolation of sample values as fetched from the referencepicture memory (557) when sub-sample exact motion vectors are in use,motion vector prediction mechanisms, and so forth.

The output samples of the aggregator (555) can be subject to variousloop filtering techniques in the loop filter unit (556). Videocompression technologies can include in-loop filter technologies thatare controlled by parameters included in the coded video sequence (alsoreferred to as coded video bitstream) and made available to the loopfilter unit (556) as symbols (521) from the parser (520), but can alsobe responsive to meta-information obtained during the decoding ofprevious (in decoding order) parts of the coded picture or coded videosequence, as well as responsive to previously reconstructed andloop-filtered sample values.

The output of the loop filter unit (556) can be a sample stream that canbe output to the render device (512) as well as stored in the referencepicture memory (557) for use in future inter-picture prediction.

Certain coded pictures, once fully reconstructed, can be used asreference pictures for future prediction. For example, once a codedpicture corresponding to a current picture is fully reconstructed andthe coded picture has been identified as a reference picture (by, forexample, the parser (520)), the current picture buffer (558) can becomea part of the reference picture memory (557), and a fresh currentpicture buffer can be reallocated before commencing the reconstructionof the following coded picture.

The video decoder (510) may perform decoding operations according to apredetermined video compression technology in a standard, such as ITU-TRec. H.265. The coded video sequence may conform to a syntax specifiedby the video compression technology or standard being used, in the sensethat the coded video sequence adheres to both the syntax of the videocompression technology or standard and the profiles as documented in thevideo compression technology or standard. Specifically, a profile canselect certain tools as the only tools available for use under thatprofile from all the tools available in the video compression technologyor standard. Also necessary for compliance can be that the complexity ofthe coded video sequence is within bounds as defined by the level of thevideo compression technology or standard. In some cases, levels restrictthe maximum picture size, maximum frame rate, maximum reconstructionsample rate (measured in, for example megasamples per second), maximumreference picture size, and so on. Limits set by levels can, in somecases, be further restricted through Hypothetical Reference Decoder(HRD) specifications and metadata for HRD buffer management signaled inthe coded video sequence.

In an embodiment, the receiver (531) may receive additional (redundant)data with the encoded video. The additional data may be included as partof the coded video sequence(s). The additional data may be used by thevideo decoder (510) to properly decode the data and/or to moreaccurately reconstruct the original video data. Additional data can bein the form of, for example, temporal, spatial, or signal noise ratio(SNR) enhancement layers, redundant slices, redundant pictures, forwarderror correction codes, and so on.

FIG. 6 shows a block diagram of a video encoder (603) according to anembodiment of the present disclosure. The video encoder (603) isincluded in an electronic device (620). The electronic device (620)includes a transmitter (640) (e.g., transmitting circuitry). The videoencoder (603) can be used in the place of the video encoder (403) in theFIG. 4 example.

The video encoder (603) may receive video samples from a video source(601) (that is not part of the electronic device (620) in the FIG. 6example) that may capture video image(s) to be coded by the videoencoder (603). In another example, the video source (601) is a part ofthe electronic device (620).

The video source (601) may provide the source video sequence to be codedby the video encoder (603) in the form of a digital video sample streamthat can be of any suitable bit depth (for example: 8 bit, 10 bit, 12bit, . . . ), any color space (for example, BT.601 Y CrCB, RGB, . . . ),and any suitable sampling structure (for example Y CrCb 4:2:0, Y CrCb4:4:4). In a media serving system, the video source (601) may be astorage device storing previously prepared video. In a videoconferencingsystem, the video source (601) may be a camera that captures local imageinformation as a video sequence. Video data may be provided as aplurality of individual pictures that impart motion when viewed insequence. The pictures themselves may be organized as a spatial array ofpixels, wherein each pixel can comprise one or more samples depending onthe sampling structure, color space, etc. in use. A person skilled inthe art can readily understand the relationship between pixels andsamples. The description below focuses on samples.

According to an embodiment, the video encoder (603) may code andcompress the pictures of the source video sequence into a coded videosequence (643) in real time or under any other time constraints asrequired by the application. Enforcing appropriate coding speed is onefunction of a controller (650). In some embodiments, the controller(650) controls other functional units as described below and isfunctionally coupled to the other functional units. The coupling is notdepicted for clarity. Parameters set by the controller (650) can includerate control related parameters (picture skip, quantizer, lambda valueof rate-distortion optimization techniques, . . . ), picture size, groupof pictures (GOP) layout, maximum motion vector search range, and soforth. The controller (650) can be configured to have other suitablefunctions that pertain to the video encoder (603) optimized for acertain system design.

In some embodiments, the video encoder (603) is configured to operate ina coding loop. As an oversimplified description, in an example, thecoding loop can include a source coder (630) (e.g., responsible forcreating symbols, such as a symbol stream, based on an input picture tobe coded, and a reference picture(s)), and a (local) decoder (633)embedded in the video encoder (603). The decoder (633) reconstructs thesymbols to create the sample data in a similar manner as a (remote)decoder also would create (as any compression between symbols and codedvideo bitstream is lossless in the video compression technologiesconsidered in the disclosed subject matter). The reconstructed samplestream (sample data) is input to the reference picture memory (634). Asthe decoding of a symbol stream leads to bit-exact results independentof decoder location (local or remote), the content in the referencepicture memory (634) is also bit exact between the local encoder andremote encoder. In other words, the prediction part of an encoder “sees”as reference picture samples exactly the same sample values as a decoderwould “see” when using prediction during decoding. This fundamentalprinciple of reference picture synchronicity (and resulting drift, ifsynchronicity cannot be maintained, for example because of channelerrors) is used in some related arts as well.

The operation of the “local” decoder (633) can be the same as of a“remote” decoder, such as the video decoder (510), which has alreadybeen described in detail above in conjunction with FIG. 5. Brieflyreferring also to FIG. 5, however, as symbols are available andencoding/decoding of symbols to a coded video sequence by an entropycoder (645) and the parser (520) can be lossless, the entropy decodingparts of the video decoder (510), including the buffer memory (515), andparser (520) may not be fully implemented in the local decoder (633).

An observation that can be made at this point is that any decodertechnology except the parsing/entropy decoding that is present in adecoder also necessarily needs to be present, in substantially identicalfunctional form, in a corresponding encoder. For this reason, thedisclosed subject matter focuses on decoder operation. The descriptionof encoder technologies can be abbreviated as they are the inverse ofthe comprehensively described decoder technologies. Only in certainareas a more detail description is required and provided below.

During operation, in some examples, the source coder (630) may performmotion compensated predictive coding, which codes an input picturepredictively with reference to one or more previously coded picture fromthe video sequence that were designated as “reference pictures.” In thismanner, the coding engine (632) codes differences between pixel blocksof an input picture and pixel blocks of reference picture(s) that may beselected as prediction reference(s) to the input picture.

The local video decoder (633) may decode coded video data of picturesthat may be designated as reference pictures, based on symbols createdby the source coder (630). Operations of the coding engine (632) mayadvantageously be lossy processes. When the coded video data may bedecoded at a video decoder (not shown in FIG. 6), the reconstructedvideo sequence typically may be a replica of the source video sequencewith some errors. The local video decoder (633) replicates decodingprocesses that may be performed by the video decoder on referencepictures and may cause reconstructed reference pictures to be stored inthe reference picture cache (634). In this manner, the video encoder(603) may store copies of reconstructed reference pictures locally thathave common content as the reconstructed reference pictures that will beobtained by a far-end video decoder (absent transmission errors).

The predictor (635) may perform prediction searches for the codingengine (632). That is, for a new picture to be coded, the predictor(635) may search the reference picture memory (634) for sample data (ascandidate reference pixel blocks) or certain metadata such as referencepicture motion vectors, block shapes, and so on, that may serve as anappropriate prediction reference for the new pictures. The predictor(635) may operate on a sample block-by-pixel block basis to findappropriate prediction references. In some cases, as determined bysearch results obtained by the predictor (635), an input picture mayhave prediction references drawn from multiple reference pictures storedin the reference picture memory (634).

The controller (650) may manage coding operations of the source coder(630), including, for example, setting of parameters and subgroupparameters used for encoding the video data.

Output of all aforementioned functional units may be subjected toentropy coding in the entropy coder (645). The entropy coder (645)translates the symbols as generated by the various functional units intoa coded video sequence, by lossless compressing the symbols according totechnologies such as Huffman coding, variable length coding, arithmeticcoding, and so forth.

The transmitter (640) may buffer the coded video sequence(s) as createdby the entropy coder (645) to prepare for transmission via acommunication channel (660), which may be a hardware/software link to astorage device which would store the encoded video data. The transmitter(640) may merge coded video data from the video coder (603) with otherdata to be transmitted, for example, coded audio data and/or ancillarydata streams (sources not shown).

The controller (650) may manage operation of the video encoder (603).During coding, the controller (650) may assign to each coded picture acertain coded picture type, which may affect the coding techniques thatmay be applied to the respective picture. For example, pictures oftenmay be assigned as one of the following picture types:

An Intra Picture (I picture) may be one that may be coded and decodedwithout using any other picture in the sequence as a source ofprediction. Some video codecs allow for different types of intrapictures, including, for example Independent Decoder Refresh (“IDR”)Pictures. A person skilled in the art is aware of those variants of Ipictures and their respective applications and features.

A predictive picture (P picture) may be one that may be coded anddecoded using intra prediction or inter prediction using at most onemotion vector and reference index to predict the sample values of eachblock.

A bi-directionally predictive picture (B Picture) may be one that may becoded and decoded using intra prediction or inter prediction using atmost two motion vectors and reference indices to predict the samplevalues of each block. Similarly, multiple-predictive pictures can usemore than two reference pictures and associated metadata for thereconstruction of a single block.

Source pictures commonly may be subdivided spatially into a plurality ofsample blocks (for example, blocks of 4×4, 8×8, 4×8, or 16×16 sampleseach) and coded on a block-by-block basis. Blocks may be codedpredictively with reference to other (already coded) blocks asdetermined by the coding assignment applied to the blocks' respectivepictures. For example, blocks of I pictures may be codednon-predictively or they may be coded predictively with reference toalready coded blocks of the same picture (spatial prediction or intraprediction). Pixel blocks of P pictures may be coded predictively, viaspatial prediction or via temporal prediction with reference to onepreviously coded reference picture. Blocks of B pictures may be codedpredictively, via spatial prediction or via temporal prediction withreference to one or two previously coded reference pictures.

The video encoder (603) may perform coding operations according to apredetermined video coding technology or standard, such as ITU-T Rec.H.265. In its operation, the video encoder (603) may perform variouscompression operations, including predictive coding operations thatexploit temporal and spatial redundancies in the input video sequence.The coded video data, therefore, may conform to a syntax specified bythe video coding technology or standard being used.

In an embodiment, the transmitter (640) may transmit additional datawith the encoded video. The source coder (630) may include such data aspart of the coded video sequence. Additional data may comprisetemporal/spatial/SNR enhancement layers, other forms of redundant datasuch as redundant pictures and slices, SEI messages, VUI parameter setfragments, and so on.

A video may be captured as a plurality of source pictures (videopictures) in a temporal sequence. Intra-picture prediction (oftenabbreviated to intra prediction) makes use of spatial correlation in agiven picture, and inter-picture prediction makes uses of the (temporalor other) correlation between the pictures. In an example, a specificpicture under encoding/decoding, which is referred to as a currentpicture, is partitioned into blocks. When a block in the current pictureis similar to a reference block in a previously coded and still bufferedreference picture in the video, the block in the current picture can becoded by a vector that is referred to as a motion vector. The motionvector points to the reference block in the reference picture, and canhave a third dimension identifying the reference picture, in casemultiple reference pictures are in use.

In some embodiments, a bi-prediction technique can be used in theinter-picture prediction. According to the bi-prediction technique, tworeference pictures, such as a first reference picture and a secondreference picture that are both prior in decoding order to the currentpicture in the video (but may be in the past and future, respectively,in display order) are used. A block in the current picture can be codedby a first motion vector that points to a first reference block in thefirst reference picture, and a second motion vector that points to asecond reference block in the second reference picture. The block can bepredicted by a combination of the first reference block and the secondreference block.

Further, a merge mode technique can be used in the inter-pictureprediction to improve coding efficiency.

According to some embodiments of the disclosure, predictions, such asinter-picture predictions and intra-picture predictions are performed inthe unit of blocks. For example, according to the HEVC standard, apicture in a sequence of video pictures is partitioned into coding treeunits (CTU) for compression, the CTUs in a picture have the same size,such as 64×64 pixels, 32×32 pixels, or 16×16 pixels. In general, a CTUincludes three coding tree blocks (CTBs), which are one luma CTB and twochroma CTBs. Each CTU can be recursively quadtree split into one ormultiple coding units (CUs). For example, a CTU of 64×64 pixels can besplit into one CU of 64×64 pixels, or 4 CUs of 32×32 pixels, or 16 CUsof 16×16 pixels. In an example, each CU is analyzed to determine aprediction type for the CU, such as an inter prediction type or an intraprediction type. The CU is split into one or more prediction units (PUs)depending on the temporal and/or spatial predictability. Generally, eachPU includes a luma prediction block (PB), and two chroma PBs. In anembodiment, a prediction operation in coding (encoding/decoding) isperformed in the unit of a prediction block. Using a luma predictionblock as an example of a prediction block, the prediction block includesa matrix of values (e.g., luma values) for pixels, such as 8×8 pixels,16×16 pixels, 8×16 pixels, 16×8 pixels, and the like.

FIG. 7 shows a diagram of a video encoder (703) according to anotherembodiment of the disclosure. The video encoder (703) is configured toreceive a processing block (e.g., a prediction block) of sample valueswithin a current video picture in a sequence of video pictures, andencode the processing block into a coded picture that is part of a codedvideo sequence. In an example, the video encoder (703) is used in theplace of the video encoder (403) in the FIG. 4 example.

In an HEVC example, the video encoder (703) receives a matrix of samplevalues for a processing block, such as a prediction block of 8×8samples, and the like. The video encoder (703) determines whether theprocessing block is best coded using intra mode, inter mode, orbi-prediction mode using, for example, rate-distortion optimization.When the processing block is to be coded in intra mode, the videoencoder (703) may use an intra prediction technique to encode theprocessing block into the coded picture; and when the processing blockis to be coded in inter mode or bi-prediction mode, the video encoder(703) may use an inter prediction or bi-prediction technique,respectively, to encode the processing block into the coded picture. Incertain video coding technologies, merge mode can be an inter pictureprediction submode where the motion vector is derived from one or moremotion vector predictors without the benefit of a coded motion vectorcomponent outside the predictors. In certain other video codingtechnologies, a motion vector component applicable to the subject blockmay be present. In an example, the video encoder (703) includes othercomponents, such as a mode decision module (not shown) to determine themode of the processing blocks.

In the FIG. 7 example, the video encoder (703) includes the interencoder (730), an intra encoder (722), a residue calculator (723), aswitch (726), a residue encoder (724), a general controller (721), andan entropy encoder (725) coupled together as shown in FIG. 7.

The inter encoder (730) is configured to receive the samples of thecurrent block (e.g., a processing block), compare the block to one ormore reference blocks in reference pictures (e.g., blocks in previouspictures and later pictures), generate inter prediction information(e.g., description of redundant information according to inter encodingtechnique, motion vectors, merge mode information), and calculate interprediction results (e.g., predicted block) based on the inter predictioninformation using any suitable technique. In some examples, thereference pictures are decoded reference pictures that are decoded basedon the encoded video information.

The intra encoder (722) is configured to receive the samples of thecurrent block (e.g., a processing block), in some cases compare theblock to blocks already coded in the same picture, generate quantizedcoefficients after transform, and in some cases also intra predictioninformation (e.g., an intra prediction direction information accordingto one or more intra encoding techniques). In an example, the intraencoder (722) also calculates intra prediction results (e.g., predictedblock) based on the intra prediction information and reference blocks inthe same picture.

The general controller (721) is configured to determine general controldata and control other components of the video encoder (703) based onthe general control data. In an example, the general controller (721)determines the mode of the block, and provides a control signal to theswitch (726) based on the mode. For example, when the mode is the intramode, the general controller (721) controls the switch (726) to selectthe intra mode result for use by the residue calculator (723), andcontrols the entropy encoder (725) to select the intra predictioninformation and include the intra prediction information in thebitstream; and when the mode is the inter mode, the general controller(721) controls the switch (726) to select the inter prediction resultfor use by the residue calculator (723), and controls the entropyencoder (725) to select the inter prediction information and include theinter prediction information in the bitstream.

The residue calculator (723) is configured to calculate a difference(residue data) between the received block and prediction resultsselected from the intra encoder (722) or the inter encoder (730). Theresidue encoder (724) is configured to operate based on the residue datato encode the residue data to generate the transform coefficients. In anexample, the residue encoder (724) is configured to convert the residuedata from a spatial domain to a frequency domain, and generate thetransform coefficients. The transform coefficients are then subject toquantization processing to obtain quantized transform coefficients. Invarious embodiments, the video encoder (703) also includes a residuedecoder (728). The residue decoder (728) is configured to performinverse-transform, and generate the decoded residue data. The decodedresidue data can be suitably used by the intra encoder (722) and theinter encoder (730). For example, the inter encoder (730) can generatedecoded blocks based on the decoded residue data and inter predictioninformation, and the intra encoder (722) can generate decoded blocksbased on the decoded residue data and the intra prediction information.The decoded blocks are suitably processed to generate decoded picturesand the decoded pictures can be buffered in a memory circuit (not shown)and used as reference pictures in some examples.

The entropy encoder (725) is configured to format the bitstream toinclude the encoded block. The entropy encoder (725) is configured toinclude various information according to a suitable standard, such asthe HEVC standard. In an example, the entropy encoder (725) isconfigured to include the general control data, the selected predictioninformation (e.g., intra prediction information or inter predictioninformation), the residue information, and other suitable information inthe bitstream. Note that, according to the disclosed subject matter,when coding a block in the merge submode of either inter mode orbi-prediction mode, there is no residue information.

FIG. 8 shows a diagram of a video decoder (810) according to anotherembodiment of the disclosure. The video decoder (810) is configured toreceive coded pictures that are part of a coded video sequence, anddecode the coded pictures to generate reconstructed pictures. In anexample, the video decoder (810) is used in the place of the videodecoder (410) in the FIG. 4 example.

In the FIG. 8 example, the video decoder (810) includes an entropydecoder (871), an inter decoder (880), a residue decoder (873), areconstruction module (874), and an intra decoder (872) coupled togetheras shown in FIG. 8.

The entropy decoder (871) can be configured to reconstruct, from thecoded picture, certain symbols that represent the syntax elements ofwhich the coded picture is made up. Such symbols can include, forexample, the mode in which a block is coded (such as, for example, intramode, inter mode, bi-predicted mode, the latter two in merge submode oranother submode), prediction information (such as, for example, intraprediction information or inter prediction information) that canidentify certain sample or metadata that is used for prediction by theintra decoder (872) or the inter decoder (880), respectively, residualinformation in the form of, for example, quantized transformcoefficients, and the like. In an example, when the prediction mode isinter or bi-predicted mode, the inter prediction information is providedto the inter decoder (880); and when the prediction type is the intraprediction type, the intra prediction information is provided to theintra decoder (872). The residual information can be subject to inversequantization and is provided to the residue decoder (873).

The inter decoder (880) is configured to receive the inter predictioninformation, and generate inter prediction results based on the interprediction information.

The intra decoder (872) is configured to receive the intra predictioninformation, and generate prediction results based on the intraprediction information.

The residue decoder (873) is configured to perform inverse quantizationto extract de-quantized transform coefficients, and process thede-quantized transform coefficients to convert the residual from thefrequency domain to the spatial domain. The residue decoder (873) mayalso require certain control information (to include the QuantizerParameter (QP)), and that information may be provided by the entropydecoder (871) (data path not depicted as this may be low volume controlinformation only).

The reconstruction module (874) is configured to combine, in the spatialdomain, the residual as output by the residue decoder (873) and theprediction results (as output by the inter or intra prediction modulesas the case may be) to form a reconstructed block, that may be part ofthe reconstructed picture, which in turn may be part of thereconstructed video. It is noted that other suitable operations, such asa deblocking operation and the like, can be performed to improve thevisual quality.

It is noted that the video encoders (403), (603), and (703), and thevideo decoders (410), (510), and (810) can be implemented using anysuitable technique. In an embodiment, the video encoders (403), (603),and (703), and the video decoders (410), (510), and (810) can beimplemented using one or more integrated circuits. In anotherembodiment, the video encoders (403), (603), and (603), and the videodecoders (410), (510), and (810) can be implemented using one or moreprocessors that execute software instructions.

An Adaptive Loop Filter (ALF) with block-based filter adaption can beapplied by encoders/decoders to reduce artifacts. For a luma component,one of a plurality of filters (e.g., 25 filters) can be selected for a4×4 luma block, for example, based on a direction and activity of localgradients.

An ALF can have any suitable shape and size. Referring to FIG. 9, ALFs(910)-(911) have a diamond shape, such as a 5×5 diamond-shape for theALF (910) and a 7×7 diamond-shape for the ALF (911). In the ALF (910),elements (920)-(932) can be used in the filtering process and form adiamond shape. Seven values (e.g., C0-C6) can be used for the elements(920)-(932). In the ALF (911), elements (940)-(964) can be used in thefiltering process and form a diamond shape. Thirteen values (e.g.,C0-C12) can be used for the elements (940)-(964).

Referring to FIG. 9, in some examples, the two ALFs (910)-(911) with thediamond filter shape are used. The 5×5 diamond-shaped filter (910) canbe applied for chroma components (e.g., chroma blocks, chroma CBs), andthe 7×7 diamond-shaped filter (911) can be applied for a luma component(e.g., a luma block, a luma CB). Other suitable shape(s) and size(s) canbe used in the ALF. For example, a 9×9 diamond-shaped filter can beused.

Filter coefficients at locations indicated by the values (e.g., C0-C6 in(910) or C0-C12 in (920)) can be non-zero. Further, when the ALFincludes a clipping function, clipping values at the locations can benon-zero.

For block classification of a luma component, a 4×4 block (or lumablock, luma CB) can be categorized or classified as one of multiple(e.g., 25) classes. A classification index C can be derived based on adirectionality parameter D and a quantized value Â of an activity valueA using Eq. (1).

C=5D+Â  Eq.(1)

To calculate the directionality parameter D and the quantized value Â,gradients g_(v), g_(h), g_(d1), and g_(d2) of a vertical, a horizontal,and two diagonal directions (e.g., d1 and d2), respectively, can becalculated using 1-D Laplacian as follows.

g _(v)=Σ_(k=i−2) ^(i+3)Σ_(l=j−2) ^(j+3) V _(k,l) V_(k,l)=|2R(k,l)−R(k,l−1)−R(k,l+1)|  Eq. (2)

g _(h)=Σ_(k=i−2) ^(i+3)Σ_(l=j−2) ^(j+3) H _(k,l) H_(k,l)=|2R(k,l)−R(k−1,l)−R(k+1,l)|  Eq. (3)

g _(d1)=Σ_(k=i−2) ^(i+3)Σ_(l=j−3) ^(j+3) D1_(k,l)D1_(k,l)=|2R(k,l)−R(k−1,l−1)−R(k+1,l+1)|  Eq. (4)

g _(d2)=Σ_(k=i−2) ^(i+3)Σ_(j=j−2) ^(j+3) D2_(k,l)D2_(k,l)=|2R(k,l)−R(k−1,l+1)−R(k+1,l−1)|  Eq. (5)

where indices i and j refer to coordinates of an upper left samplewithin the 4×4 block and R(k,l) indicates a reconstructed sample at acoordinate (k,l). The directions (e.g., d1 and d2) can refer to 2diagonal directions.

To reduce complexity of the block classification described above, asubsampled 1-D Laplacian calculation can be applied. FIGS. 10A-10D showexamples of subsampled positions used for calculating the gradientsg_(v), g_(h), g_(d1), and g_(d2) of the vertical (FIG. 10A), thehorizontal (FIG. 10B), and the two diagonal directions d1 (FIG. 10C) andd2 (FIG. 10D), respectively. The same subsampled positions can be usedfor gradient calculation of the different directions. In FIG. 10A,labels ‘V’ show the subsampled positions to calculate the verticalgradient g_(v). In FIG. 10B, labels ‘H’ show the subsampled positions tocalculate the horizontal gradient g_(h). In FIG. 10C, labels ‘D1’ showthe subsampled positions to calculate the d1 diagonal gradient g_(d1).In FIG. 10D, labels ‘D2’ show the subsampled positions to calculate thed2 diagonal gradient g_(d2).

A maximum value g_(h,v) ^(max) and a minimum value g_(h,v) ^(min) of thegradients of horizontal and vertical directions g_(v) and g_(h) can beset as:

g _(h,v) ^(max)=max(g _(h) ,g _(v)),g _(h,v) ^(min)=min(g _(h) ,g_(v))  Eq. (6)

A maximum value g_(d1,d2) ^(max) and a minimum value g_(d1,d2) ^(min) ofthe gradients of two diagonal directions g_(d1) and g_(d2) can be setas:

g _(d1,d2) ^(max)=max(g _(d1) ,g _(d2)),g _(d1,d2) ^(min)=min(g _(d1) ,g_(d2))  Eq. (7)

The directionality parameter D can be derived based on the above valuesand two thresholds t₁ and t₂ as below.Step 1. If (1) g_(h,v) ^(max)≤t₁·g_(h,v) ^(min) and (2) g_(d1,d2)^(max)≤t₁·g_(d1,d2) ^(min) are true, D is set to 0.Step 2. If g_(h,v) ^(max)/g_(h,v) ^(min)>g_(d1,d2) ^(max)/g_(d1,d2)^(min), continue to Step 3; otherwise continue to Step 4.Step 3. If g_(h,v) ^(max)>t₂·g_(h,v) ^(min), D is set to 2; otherwise Dis set to 1.Step 4. If g_(d1,d2) ^(max)>t₂·g_(d1,d2) ^(min), D is set to 4;otherwise D is set to 3.

The activity value A can be calculated as:

A=Σ _(k=i−2) ^(i+3)Σ_(l=j−2) ^(j+3)(V _(k,l) +H _(k,l))  Eq. (8)

A can be further quantized to a range of 0 to 4, inclusively, and thequantized value is denoted as Â.

For chroma components in a picture, no block classification is applied,and thus a single set of ALF coefficients can be applied for each chromacomponent.

Geometric transformations can be applied to filter coefficients andcorresponding filter clipping values (also referred to as clippingvalues). Before filtering a block (e.g., a 4×4 luma block), geometrictransformations such as rotation or diagonal and vertical flipping canbe applied to the filter coefficients f(k,l) and the correspondingfilter clipping values c(k,l), for example, depending on gradient values(e.g., g_(v), g_(h), g_(d1), and/or g_(d2)) calculated for the block.The geometric transformations applied to the filter coefficients f(k,l)and the corresponding filter clipping values c(k,l) can be equivalent toapplying the geometric transformations to samples in a region supportedby the filter. The geometric transformations can make different blocksto which an ALF is applied more similar by aligning the respectivedirectionality.

Three geometric transformations, including a diagonal flip, a verticalflip, and a rotation can be performed as described by Eqs. (9)-(11),respectively.

f _(D)(k,l)=f(l,k),c _(D)(k,l)=c(l,k),  Eq. (9)

f _(V)(k,l)=f(k,K−l−1),c _(V)(k,l)=c(k,K−l−1)  Eq. (10)

f _(R)(k,l)=f(K−l−1,k),c _(R)(k,l)=c(K−l−1,k)  Eq. (11)

where K is a size of the ALF or the filter, and 0≤k,l≤K−1 arecoordinates of coefficients. For example, a location (0,0) is at anupper left corner and a location (K−1, K−1) is at a lower right cornerof the filter f or a clipping value matrix (or clipping matrix) c. Thetransformations can be applied to the filter coefficients f(k,l) and theclipping values c(k,l) depending on the gradient values calculated forthe block. An example of a relationship between the transformation andthe four gradients are summarized in Table 1.

TABLE 1 Mapping of the gradient calculated for a block and thetransformation Gradient values Transformation g_(d2) < g_(d1) and g_(h)< g_(v) No transformation g_(d2) < g_(d1) and g_(v) < g_(h) Diagonalflip g_(d1) < g_(d2) and g_(h) < g_(v) Vertical flip g_(d1) < g_(d2) andg_(v) < g_(h) Rotation

In some embodiments, ALF filter parameters are signaled in an AdaptationParameter Set (APS) for a picture. In the APS, one or more sets (e.g.,up to 25 sets) of luma filter coefficients and clipping value indexescan be signaled. In an example, a set of the one or more sets caninclude luma filter coefficients and one or more clipping value indexes.One or more sets (e.g., up to 8 sets) of chroma filter coefficients andclipping value indexes can be signaled. To reduce signaling overhead,filter coefficients of different classifications (e.g., having differentclassification indices) for luma components can be merged. In a sliceheader, indices of the APSs used for a current slice can be signaled.

In an embodiment, a clipping value index (also referred to as clippingindex) can be decoded from the APS. The clipping value index can be usedto determine a corresponding clipping value, for example, based on arelationship between the clipping value index and the correspondingclipping value. The relationship can be pre-defined and stored in adecoder. In an example, the relationship is described by a table, suchas a luma table (e.g., used for a luma CB) of the clipping value indexand the corresponding clipping value, a chroma table (e.g., used for achroma CB) of the clipping value index and the corresponding clippingvalue. The clipping value can be dependent of a bit depth B. The bitdepth B can refer to an internal bit depth, a bit depth of reconstructedsamples in a CB to be filtered, or the like. In some examples, a table(e.g., a luma table, a chroma table) is obtained using Eq. (12).

$\begin{matrix}{{{AlfClip} = \left\{ {{{round}\left( 2^{B\frac{N - n + 1}{N}} \right){for}n} \in \left\lbrack {1¨N} \right\rbrack} \right\}},} & {{Eq}.(12)}\end{matrix}$

where AlfClip is the clipping value, B is the bit depth (e.g.,bitDepth), N (e.g., N=4) is a number of allowed clipping values, and(n−1) is the clipping value index (also referred to as clipping index orclipIdx). Table 2 shows an example of a table obtained using Eq. (12)with N=4. The clipping index (n−1) can be 0, 1, 2, and 3 in Table 2, andn can be 1, 2, 3, and 4, respectively. Table 2 can be used for lumablocks or chroma blocks.

TABLE 2 AlfClip can depend on the bit depth B and clipIdx clipIdxbitDepth 0 1 2 3 8 255 64 16 4 9 511 108 23 5 10 1023 181 32 6 11 2047304 45 7 12 4095 512 64 8 13 8191 861 91 10 14 16383 1448 128 11 1532767 2435 181 13 16 65535 4096 256 16

In a slice header for a current slice, one or more APS indices (e.g., upto 7 APS indices) can be signaled to specify luma filter sets that canbe used for the current slice. The filtering process can be controlledat one or more suitable levels, such as a picture level, a slice level,a CTB level, and/or the like. In an embodiment, the filtering processcan be further controlled at a CTB level. A flag can be signaled toindicate whether the ALF is applied to a luma CTB. The luma CTB canchoose a filter set among a plurality of fixed filter sets (e.g., 16fixed filter sets) and the filter set(s) (also referred to as signaledfilter set(s)) that are signaled in the APSs. A filter set index can besignaled for the luma CTB to indicate the filter set (e.g., the filterset among the plurality of fixed filter sets and the signaled filterset(s)) to be applied. The plurality of fixed filter sets can bepre-defined and hard-coded in an encoder and a decoder, and can bereferred to as pre-defined filter sets.

For a chroma component, an APS index can be signaled in the slice headerto indicate the chroma filter sets to be used for the current slice. Atthe CTB level, a filter set index can be signaled for each chroma CTB ifthere is more than one chroma filter set in the APS.

The filter coefficients can be quantized with a norm equal to 128. Inorder to decrease the multiplication complexity, a bitstream conformancecan be applied so that the coefficient value of the non-central positioncan be in a range of −27 to 27-1, inclusive. In an example, the centralposition coefficient is not signaled in the bitstream and can beconsidered as equal to 128.

In some embodiments, the syntaxes and semantics of clipping index andclipping values are defined as follows:

alf_luma_clip_idx[sfIdx][j] can be used to specify the clipping index ofthe clipping value to use before multiplying by the j-th coefficient ofthe signaled luma filter indicated by sfIdx. A requirement of bitstreamconformance can include that the values of alf_luma_clip_idx[sfIdx][j]with sfIdx=0 to alf_luma_num_filters_signalled_minus1 and j=0 to 11shall be in the range of 0 to 3, inclusive.The luma filter clipping values AlfClipL[adaptation_parameter_set_id]with elements AlfClipL[adaptation_parameter_set_id][filtIdx][j], withfiltIdx=0 to NumAlfFilters−1 and j=0 to 11 can be derived as specifiedin Table 2 depending on bitDepth set equal to BitDepthY and clipIdx setequal to alf_luma_clip_idx[alf_luma_coeff_delta_idx[filtIdx] ][j].alf_chroma_clip_idx[altIdx][j] can be used to specify the clipping indexof the clipping value to use before multiplying by the j-th coefficientof the alternative chroma filter with index altldx. A requirement ofbitstream conformance can include that the values ofalf_chroma_clip_idx[altIdx][j] with altIdx=0 toalf_chroma_num_alt_filters_minus1, j=0 to 5 shall be in the range of 0to 3, inclusive.The chroma filter clipping valuesAlfClipC[adaptation_parameter_set_id][altIdx] with elementsAlfClipC[adaptation_parameter_set_id][altIdx][j], with altIdx=0 toalf_chroma_num_alt_filters_minus1, j=0 to 5 can be derived as specifiedin Table 2 depending on bitDepth set equal to BitDepthC and clipIdx setequal to alf_chroma_clip_idx[altIdx][j].

In an embodiment, the filtering process can be described as below. At adecoder side, when the ALF is enabled for a CTB, a sample R(i,j) withina CU (or CB) can be filtered, resulting in a filtered sample valueR′(i,j) as shown below using Eq. (13). In an example, each sample in theCU is filtered.

$\begin{matrix}\left. {{{{R^{\prime}\left( {i,j} \right)} = {{R\left( {i,j} \right)} + \left( \left( {{\sum\limits_{k \neq 0}{\sum\limits_{l \neq 0}{{f\left( {k,l} \right)} \times {K\left( {{{R\left( {{i + k},{j + l}} \right)} - {R\left( {i,j} \right)}},{c\left( {k,l} \right)}} \right)}}}} + 64} \right) \right.}}}7} \right) & {{Eq}.(13)}\end{matrix}$

where f(k,l) denotes the decoded filter coefficients, K(x,y) is aclipping function, and c(k,l) denotes the decoded clipping parameters(or clipping values). The variables k and 1 can vary between −L/2 andL/2 where L denotes a filter length. The clipping function K(x,y)=min(y, max(−y, x)) corresponds to a clipping function Clip3 (−y, y, x). Byincorporating the clipping function K(x,y), the loop filtering method(e.g., ALF) becomes a non-linear process, and can be referred to anonlinear ALF.

In the nonlinear ALF, multiple sets of clipping values can be providedin Table 3. In an example, a luma set includes four clipping values{1024, 181, 32, 6}, and a chroma set includes 4 clipping values {1024,161, 25, 4}. The four clipping values in the luma set can be selected byapproximately equally splitting, in a logarithmic domain, a full range(e.g., 1024) of the sample values (coded on 10 bits) for a luma block.The range can be from 4 to 1024 for the chroma set.

TABLE 3 Examples of clipping values INTRA/INTER tile group LUMA {1024,181, 32, 6} CHROMA {1024, 161, 25, 4}

The selected clipping values can be coded in an “alf_data” syntaxelement as follows: a suitable encoding scheme (e.g., a Golomb encodingscheme) can be used to encode a clipping index corresponding to theselected clipping value such as shown in Table 3. The encoding schemecan be the same encoding scheme used for encoding the filter set index.

In an embodiment, a virtual boundary filtering process can be used toreduce a line buffer requirement of the ALF. Accordingly, modified blockclassification and filtering can be employed for samples near CTUboundaries (e.g., a horizontal CTU boundary). A virtual boundary (1130)can be defined as a line by shifting a horizontal CTU boundary (1120) by“N_(samples)” samples, as shown in FIG. 11A, where N_(samples) can be apositive integer. In an example, N_(samples) is equal to 4 for a lumacomponent, and N_(samples) is equal to 2 for a chroma component.

Referring to FIG. 11A, a modified block classification can be appliedfor a luma component. In an example, for the 1D Laplacian gradientcalculation of a 4×4 block (1110) above the virtual boundary (1130),only samples above the virtual boundary (1130) are used. Similarly,referring to FIG. 11B, for a 1D Laplacian gradient calculation of a 4×4block (1111) below a virtual boundary (1131) that is shifted from a CTUboundary (1121), only samples below the virtual boundary (1131) areused. The quantization of an activity value A can be accordingly scaledby taking into account a reduced number of samples used in the 1DLaplacian gradient calculation.

For a filtering processing, a symmetric padding operation at virtualboundaries can be used for both a luma component and a chroma component.FIGS. 12A-12F illustrate examples of such modified ALF filtering for aluma component at virtual boundaries. When a sample being filtered islocated below a virtual boundary, neighboring samples that are locatedabove the virtual boundary can be padded. When a sample being filteredis located above a virtual boundary, neighboring samples that arelocated below the virtual boundary can be padded. Referring to FIG. 12A,a neighboring sample C0 can be padded with a sample C2 that is locatedbelow a virtual boundary (1210). Referring to FIG. 12B, a neighboringsample C0 can be padded with a sample C2 that is located above a virtualboundary (1220). Referring to FIG. 12C, neighboring samples C1-C3 can bepadded with samples C5-C7, respectively, that are located below avirtual boundary (1230). Referring to FIG. 12D, neighboring samplesC1-C3 can be padded with samples C5-C7, respectively, that are locatedabove a virtual boundary (1240). Referring to FIG. 12E, neighboringsamples C4-C8 can be padded with samples C10, C11, C12, C11, and C10,respectively, that are located below a virtual boundary (1250).Referring to FIG. 12F, neighboring samples C4-C8 can be padded withsamples C10, C11, C12, C11, and C10, respectively, that are locatedabove a virtual boundary (1260).

In some examples, the above description can be suitably adapted whensample(s) and neighboring sample(s) are located to the left (or to theright) and to the right (or to the left) of a virtual boundary.

A cross-component filtering process can apply cross-component filters,such as cross-component adaptive loop filters (CC-ALFs). Thecross-component filter can use luma sample values of a luma component(e.g., a luma CB) to refine a chroma component (e.g., a chroma CBcorresponding to the luma CB). In an example, the luma CB and the chromaCB are included in a CU.

FIG. 13 shows cross-component filters (e.g., CC-ALFs) used to generatechroma components according to an embodiment of the disclosure. In someexamples, FIG. 13 shows filtering processes for a first chroma component(e.g., a first chroma CB), a second chroma component (e.g., a secondchroma CB), and a luma component (e.g., a luma CB). The luma componentcan be filtered by a sample adaptive offset (SAO) filter (1310) togenerate a SAO filtered luma component (1341). The SAO filtered lumacomponent (1341) can be further filtered by an ALF luma filter (1316) tobecome a filtered luma CB (1361) (e.g., ‘Y’).

The first chroma component can be filtered by a SAO filter (1312) and anALF chroma filter (1318) to generate a first intermediate component(1352). Further, the SAO filtered luma component (1341) can be filteredby a cross-component filter (e.g., CC-ALF) (1321) for the first chromacomponent to generate a second intermediate component (1342).Subsequently, a filtered first chroma component (1362) (e.g., ‘Cb’) canbe generated based on at least one of the second intermediate component(1342) and the first intermediate component (1352). In an example, thefiltered first chroma component (1362) (e.g., ‘Cb’) can be generated bycombining the second intermediate component (1342) and the firstintermediate component (1352) with an adder (1322). The cross-componentadaptive loop filtering process for the first chroma component caninclude a step performed by the CC-ALF (1321) and a step performed by,for example, the adder (1322).

The above description can be adapted to the second chroma component. Thesecond chroma component can be filtered by a SAO filter (1314) and theALF chroma filter (1318) to generate a third intermediate component(1353). Further, the SAO filtered luma component (1341) can be filteredby a cross-component filter (e.g., a CC-ALF) (1331) for the secondchroma component to generate a fourth intermediate component (1343).Subsequently, a filtered second chroma component (1363) (e.g., ‘Cr’) canbe generated based on at least one of the fourth intermediate component(1343) and the third intermediate component (1353). In an example, thefiltered second chroma component (1363) (e.g., ‘Cr’) can be generated bycombining the fourth intermediate component (1343) and the thirdintermediate component (1353) with an adder (1332). In an example, thecross-component adaptive loop filtering process for the second chromacomponent can include a step performed by the CC-ALF (1331) and a stepperformed by, for example, the adder (1332).

A cross-component filter (e.g., the CC-ALF (1321), the CC-ALF (1331))can operate by applying a linear filter having any suitable filter shapeto the luma component (or a luma channel) to refine each chromacomponent (e.g., the first chroma component, the second chromacomponent).

FIG. 14 shows an example of a filter (1400) according to an embodimentof the disclosure. The filter (1400) can include non-zero filtercoefficients and zero filter coefficients. The filter (1400) has adiamond shape (1420) formed by filter coefficients (1410) (indicated bycircles having black fill). In an example, the non-zero filtercoefficients in the filter (1400) are included in the filtercoefficients (1410), and filter coefficients not included in the filtercoefficients (1410) are zero. Thus, the non-zero filter coefficients inthe filter (1400) are included in the diamond shape (1420), and thefilter coefficients not included in the diamond shape (1420) are zero.In an example, a number of the filter coefficients of the filter (1400)is equal to a number of the filter coefficients (1410), which is 18 inthe example shown in FIG. 14.

The CC-ALF can include any suitable filter coefficients (also referredto as the CC-ALF filter coefficients). Referring back to FIG. 13, theCC-ALF (1321) and the CC-ALF (1331) can have a same filter shape, suchas the diamond shape (1420) shown in FIG. 14, and a same number offilter coefficients. In an example, values of the filter coefficients inthe CC-ALF (1321) are different from values of the filter coefficientsin the CC-ALF (1331).

In general, filter coefficients (e.g., non-zero filter coefficients) ina CC-ALF can be transmitted, for example, in the APS. In an example, thefilter coefficients can be scaled by a factor (e.g., 2¹⁰) and can berounded for a fixed point representation. Application of a CC-ALF can becontrolled on a variable block size and signaled by a context-coded flag(e.g., a CC-ALF enabling flag) received for each block of samples. Thecontext-coded flag, such as the CC-ALF enabling flag, can be signaled atany suitable level, such as a block level. The block size along with theCC-ALF enabling flag can be received at a slice-level for each chromacomponent. In some examples, block sizes (in chroma samples) 16×16,32×32, and 64×64 can be supported.

In general, a luma block can correspond to chroma block(s), such as twochroma blocks. A number of samples in each of the chroma block(s) can beless than a number of samples in the luma block. A chroma subsamplingformat (also referred to as a chroma subsampling format, e.g., specifiedby chroma_format_idc) can indicate a chroma horizontal subsamplingfactor (e.g., SubWidthC) and a chroma vertical subsampling factor (e.g.,SubHeightC) between each of the chroma block(s) and the correspondingluma block. In an example, the chroma subsampling format is 4:2:0, andthus the chroma horizontal subsampling factor (e.g., SubWidthC) and thechroma vertical subsampling factor (e.g., SubHeightC) are 2, as shown inFIGS. 15A-15B. In an example, the chroma subsampling format is 4:2:2,and thus the chroma horizontal subsampling factor (e.g., SubWidthC) is2, and the chroma vertical subsampling factor (e.g., SubHeightC) is 1.In an example, the chroma subsampling format is 4:4:4, and thus thechroma horizontal subsampling factor (e.g., SubWidthC) and the chromavertical subsampling factor (e.g., SubHeightC) are 1. A chroma sampletype (also referred to as a chroma sample position) can indicate arelative position of a chroma sample in the chroma block with respect toat least one corresponding luma sample in the luma block.

FIGS. 15A-15B show exemplary locations of chroma samples relative toluma samples according to embodiments of the disclosure. Referring toFIG. 15A, the luma samples (1501) are located in rows (1511)-(1518). Theluma samples (1501) shown in FIG. 15A can represent a portion of apicture. In an example, a luma block (e.g., a luma CB) includes the lumasamples (1501). The luma block can correspond to two chroma blockshaving the chroma subsampling format of 4:2:0. In an example, eachchroma block includes chroma samples (1503). Each chroma sample (e.g.,the chroma sample (1503(1)) corresponds to four luma samples (e.g., theluma samples (1501(1))-(1501(4)). In an example, the four luma samplesare the top-left sample (1501(1)), the top-right sample (1501(2)), thebottom-left sample (1501(3)), and the bottom-right sample (1501(4)). Thechroma sample (e.g., (1503(1))) is located at a left center positionthat is between the top-left sample (1501(1)) and the bottom-left sample(1501(3)), and a chroma sample type of the chroma block having thechroma samples (1503) can be referred to as a chroma sample type 0. Thechroma sample type 0 indicates a relative position 0 corresponding tothe left center position in the middle of the top-left sample (1501(1))and the bottom-left sample (1501(3)). The four luma samples (e.g.,(1501(1))-(1501(4))) can be referred to as neighboring luma samples ofthe chroma sample (1503)(1).

In an example, each chroma block includes chroma samples (1504). Theabove description with reference to the chroma samples (1503) can beadapted to the chroma samples (1504), and thus detailed descriptions canbe omitted for purposes of brevity. Each of the chroma samples (1504)can be located at a center position of four corresponding luma samples,and a chroma sample type of the chroma block having the chroma samples(1504) can be referred to as a chroma sample type 1. The chroma sampletype 1 indicates a relative position 1 corresponding to the centerposition of the four luma samples (e.g., (1501(1))-(1501(4))). Forexample, one of the chroma samples (1504) can be located at a centerportion of the luma samples (1501(1))-(1501(4)).

In an example, each chroma block includes chroma samples (1505). Each ofthe chroma samples (1505) can be located at a top left position that isco-located with the top-left sample of the four corresponding lumasamples (1501), and a chroma sample type of the chroma block having thechroma samples (1505) can be referred to as a chroma sample type 2.Accordingly, each of the chroma samples (1505) is co-located with thetop left sample of the four luma samples (1501) corresponding to therespective chroma sample. The chroma sample type 2 indicates a relativeposition 2 corresponding to the top left position of the four lumasamples (1501). For example, one of the chroma samples (1505) can belocated at a top left position of the luma samples (1501(1))-(1501(4)).

In an example, each chroma block includes chroma samples (1506). Each ofthe chroma samples (1506) can be located at a top center positionbetween a corresponding top-left sample and a corresponding top-rightsample, and a chroma sample type of the chroma block having the chromasamples (1506) can be referred to as a chroma sample type 3. The chromasample type 3 indicates a relative position 3 corresponding to the topcenter position between the top-left sample (and the top-right sample.For example, one of the chroma samples (1506) can be located at a topcenter position of the luma samples (1501(1))-(1501(4)).

In an example, each chroma block includes chroma samples (1507). Each ofthe chroma samples (1507) can be located at a bottom left position thatis co-located with the bottom-left sample of the four corresponding lumasamples (1501), and a chroma sample type of the chroma block having thechroma samples (1507) can be referred to as a chroma sample type 4.Accordingly, each of the chroma samples (1507) is co-located with thebottom left sample of the four luma samples (1501) corresponding to therespective chroma sample. The chroma sample type 4 indicates a relativeposition 4 corresponding to the bottom left position of the four lumasamples (1501). For example, one of the chroma samples (1507) can belocated at a bottom left position of the luma samples(1501(1))-(1501(4)).

In an example, each chroma block includes chroma samples (1508). Each ofthe chroma samples (1508) is located at a bottom center position betweenthe bottom-left sample and the bottom-right sample, and a chroma sampletype of the chroma block having the chroma samples (1508) can bereferred to as a chroma sample type 5. The chroma sample type 5indicates a relative position 5 corresponding to the bottom centerposition between the bottom-left sample and the bottom-right sample ofthe four luma samples (1501). For example, one of the chroma samples(1508) can be located between the bottom-left sample and thebottom-right sample of the luma samples (1501(1))-(1501(4)).

In general, any suitable chroma sample type can be used for a chromasubsampling format. The chroma sample types 0-5 are exemplary chromasample types described with the chroma subsampling format 4:2:0.Additional chroma sample types may be used for the chroma subsamplingformat 4:2:0. Further, other chroma sample types and/or variations ofthe chroma sample types 0-5 can be used for other chroma subsamplingformats, such as 4:2:2, 4:4:4, or the like. In an example, a chromasample type combining the chroma samples (1505) and (1507) is used forthe chroma subsampling format 4:2:2.

In an example, the luma block is considered to have alternating rows,such as the rows (1511)-(1512) that include the top two samples (e.g.,(1501(1))-(150)(2))) of the four luma samples (e.g.,(1501(1))-(1501(4))) and the bottom two samples (e.g.,(1501(3))-(1501(4))) of the four luma samples (e.g.,(1501(1)-(1501(4))), respectively. Accordingly, the rows (1511), (1513),(1515), and (1517) can be referred to as current rows (also referred toas a top field), and the rows (1512), (1514), (1516), and (1518) can bereferred to as next rows (also referred to as a bottom field). The fourluma samples (e.g., (1501(1))-(1501(4))) are located at the current row(e.g., (1511)) and the next row (e.g., (1512)). The relative positions2-3 are located in the current rows, the relative positions 0-1 arelocated between each current row and the respective next row, and therelative positions 4-5 are located in the next rows.

The chroma samples (1503), (1504), (1505), (1506), (1507), or (1508) arelocated in rows (1551)-(1554) in each chroma block. Specific locationsof the rows (1551)-(1554) can depend on the chroma sample type of thechroma samples. For example, for the chroma samples (1503)-(1504) havingthe respective chroma sample types 0-1, the row (1551) is locatedbetween the rows (1511)-(1512). For the chroma samples (1505)-(1506)having the respective the chroma sample types 2-3, the row (1551) isco-located with the current row (1511). For the chroma samples(1507)-(1508) having the respective the chroma sample types 4-5, the row(1551) is co-located with the next row (1512). The above descriptionscan be suitably adapted to the rows (1552)-(1554), and the detaileddescriptions are omitted for purposes of brevity.

Any suitable scanning method can be used for displaying, storing, and/ortransmitting the luma block and the corresponding chroma block(s)described above in FIG. 15A. In an example, progressive scanning isused.

An interlaced scan can be used, as shown in FIG. 15B. As describedabove, the chroma subsampling format is 4:2:0 (e.g., chroma_format_idcis equal to 1). In an example, a variable chroma location type (e.g.,ChromaLocType) indicates the current rows (e.g., ChromaLocType ischroma_sample_loc_type_top_field) or the next rows (e.g., ChromaLocTypeis chroma_sample_loc_type_bottom_field). The current rows (1511),(1513), (1515), and (1517) and the next rows (1512), (1514), (1516), and(1518) can be scanned separately, for example, the current rows (1511),(1513), (1515), and (1517) can be scanned first followed by the nextrows (1512), (1514), (1516), and (1518) being scanned. The current rowscan include the luma samples (1501) while the next rows can include theluma samples (1502).

Similarly, the corresponding chroma block can be interlaced scanned. Therows (1551) and (1553) including the chroma samples (1503), (1504),(1505), (1506), (1507), or (1508) with no fill can be referred to ascurrent rows (or current chroma rows), and the rows (1552) and (1554)including the chroma samples (1503), (1504), (1505), (1506), (1507), or(1508) with gray fill can be referred to as next rows (or next chromarows). In an example, during the interlaced scan, the rows (1551) and(1553) are scanned first followed by scanning the rows (1552) and(1554).

The diamond filter shape (1420) in FIG. 14 is designed for the chromasubsampling format of 4:2:0 and the chroma sample type 0 (e.g., a chromarow is between two luma rows), which may not be efficient for otherchroma sample types (e.g., the chroma sample types 1-5) and other chromasubsampling formats (e.g., 4:2:2 and 4:4:4).

Coded information of a chroma block or a chroma CB (e.g., the firstchroma CB or the second chroma CB in FIG. 13) can be decoded from acoded video bitstream. The coded information can indicate that across-component filter is applied to the chroma CB. The codedinformation can further include a chroma subsampling format and a chromasample type. As described above, the chroma subsampling format canindicate a chroma horizontal subsampling factor and a chroma verticalsubsampling factor between the chroma CB and a corresponding luma CB(e.g., the luma CB in FIG. 13). The chroma sample type can indicate arelative position of a chroma sample with respect to at least onecorresponding luma sample in the luma CB. In an example, the chromasample type is signaled in the coded video bitstream. The chroma sampletype can be signaled at any suitable level, such as in a sequenceparameter set (SPS).

According to aspects of the disclosure, a filter shape of across-component filter (e.g., the CC-ALF (1321)) in the cross-componentfiltering process can be determined based on at least one of the chromasubsampling format and the chroma sample type. Further, a firstintermediate CB (e.g., the intermediate component (1342)) can begenerated by applying the cross-component filter having the determinedfilter shape to the corresponding luma CB (e.g., the SAO filtered lumacomponent (1341)). A second intermediate CB (e.g., the intermediatecomponent (1352)) can be generated by applying a loop filter (e.g., theALF (1318)) to the chroma CB (e.g., the SAO filtered first chroma CB). Afiltered chroma CB (e.g., the filtered first chroma component (1362)(e.g., ‘Cb’) in FIG. 13) can be determined based on the firstintermediate CB and the second intermediate CB. As described above, thecross-component filter can be a CC-ALF and the loop filter can be anALF.

The chroma sample type of the chroma block can be indicated in the codedbitstream when the CC-ALF is used. The filter shape of the CC-ALF can bedependent on the chroma subsampling format (e.g., chroma_format_idc) ofthe chroma block, the chroma sample type, and/or the like.

FIG. 16 shows exemplary cross-component filters (e.g., CC-ALFs)(1601)-(1603) having respective filter shapes (1621)-(1623) according toembodiments of the disclosure. Referring to FIGS. 14 and 16, the filtershapes (1420) and (1621)-(1623) can be used for the CC-ALF based on thechroma sample type of the chroma block, for example, when the chromasubsampling format of 4:2:0.

According to aspects of the disclosure, the chroma sample type can beone of the six chroma sample types 0-5 indicating the six relativepositions 0-5, respectively. The six relative positions 0-5 cancorrespond to the left-center position, the center position, thetop-left position, the top-center position, the bottom-left position,and the bottom-center position of the four luma samples (e.g.,(1501(1))-(1501(4))), respectively, such as shown in FIG. 15A. Thefilter shape of the cross-component filter can be determined based onthe chroma sample type.

When the chroma sample type of the chroma block is the chroma sampletype 0, the filter (1400) having the filter shape (1420) in FIG. 14 canbe used in the cross-component filter (e.g., the CC-ALF).

In an example, when the chroma sample type of the chroma block is thechroma sample type 1, a square filter shape (e.g., a 4×4 square filtershape, a 2×2 square filter shape) can be used in the cross-componentfilter (e.g., the CC-ALF). A chroma sample (e.g., (1504(1)) in FIG. 15A)being cross-component filtered can be located in a center of the squarefilter shape as the chroma sample (e.g., (1504(1))) is located in thecenter of four corresponding luma samples (e.g., (1501(1))-(1501(4))).

In an example, when the chroma sample type of the chroma block is thechroma sample type 2, a diamond filter shape (e.g., the 5×5 diamondfilter shape (1621) of the filter (1601) or the 3×3 diamond filter shape(1622) of the filter (1602)) can be used in the CC-ALF.

In an example, when the chroma sample type of the chroma block is thechroma sample type 3, the diamond filter shape (1623) of the filter(1603) can be used in the CC-ALF. Referring to FIGS. 14 and 16, thediamond filter shape (1623) is a geometric transformation (e.g., a 90°rotation) of the filter shape (1420).

In an example, when the chroma sample type of the chroma block is thechroma sample type 4, a filter shape used in the CC-ALF can be identicalor similar to that (e.g., the diamond filter shape (1621) or (1622))used for the chroma sample type 2. Thus, the filter shape for the chromasample type 4 can be a diamond filter shape, such as the diamond filtershape (1621) or (1622) shifted vertically.

In an example, when the chroma sample type of the chroma block is thechroma sample type 5, a filter shape used in the CC-ALF can be identicalor similar to that (e.g., the diamond filter shape (1623)) used for thechroma sample type 3. Thus, the filter shape for the chroma sample type5 can be a diamond filter shape, such as the diamond filter shape (1623)shifted vertically.

In an embodiment, the number of filter coefficients is signaled in thecoded video bitstream, such as in an APS. Referring to FIGS. 14 and 16,filter coefficients (1611) can form the diamond shape (1621), and otherfilter coefficients not included in, or otherwise excluded from, thefilter coefficients (1611) are zero. Accordingly, the number of filtercoefficients for the filter (1601) can refer to the number of filtercoefficients in the filter shape (1621), and thus can be equal to thenumber (e.g., 13) of filter coefficients (1611). Similarly, the numberof filter coefficients of the filter (1602) can refer to the number offilter coefficients in the filter shape (1622), and thus can be equal tothe number (e.g., 5) of filter coefficients (1612). The number of filtercoefficients of the filter (1603) can refer to the number of filtercoefficients in the filter shape (1623), and thus can be equal to thenumber (e.g., 18) of filter coefficients (1613). Similarly, the numberof filter coefficients of the filter (1400) can be 18.

Different filter shapes can have different numbers of coefficients.Thus, in some examples, the filter shape can be determined based on thenumber of filter coefficients. For example, when the number of filtercoefficients is 16, the filter shape can be determined to be a 4×4square filter shape.

According to aspects of the disclosure, the number of filtercoefficients of the cross-component filter can be signaled in the codedvideo bitstream. Further, the filter shape of the cross-component filtercan be determined based on the number of filter coefficients and the atleast one of the chroma subsampling format and the chroma sample type.

In an embodiment, a cross-component linear model (CCLM) flag canindicate the chroma sample type. Accordingly, the filter shape of theCC-ALF may be dependent on the CCLM flag. In an example, the chromasample type indicated by the CCLM flag is the chroma sample type 0 or 2,and thus the filter shape can be the filter shape (1420) or the diamondfilter shape (e.g., the 5×5 diamond filter shape (1621) or the 3×3diamond filter shape (1622)).

In an embodiment, the CCLM flag (e.g., a sps_cclm_colocated_chroma flag)is signaled in the coded video bitstream, for example, in the SPS. In anexample, the CCLM flag (e.g., the sps_cclm_colocated_chroma_flag)indicates whether a top-left down-sampled luma sample in a CCLM intraprediction is collocated with a top-left luma sample. As describedabove, the chroma sample type indicated by the CCLM flag can be thechroma sample type 0 or 2. The filter shape of the CC-ALF may bedependent on the sps_cclm_colocated_chroma_flag or similar information(e.g., information indicating whether the top-left down-sampled lumasample in the CCLM intra prediction is collocated with the top-left lumasample) for CCLM signaled in the SPS.

In some examples, the cross-component filter (e.g., the CC-ALF) includesa large number (e.g., 18) of multiplications per chroma sample (e.g., aCb chroma sample or a Cr chroma sample), thus having a high cost, forexample, in calculation complexity. The number of multiplications isbased on a number of filter coefficients (e.g., 18 filter coefficientsin the filters (1400) and (1603), 13 filter coefficients in the filter(1601), and 5 filter coefficients in the filter (1602)) in the CC-ALF.For example, the number of multiplications is equal to the number offilter coefficients in the CC-ALF. According to aspects of thedisclosure, a number of bits representing the CC-ALF filter coefficientsof the CC-ALF can be constrained to be less than or equal to K bits. Kcan be a positive integer, such as 8. Thus, the CC-ALF filtercoefficients can be included in a range of [−2^(K−1) to 2^(K−1)−1]. Therange of the CC-ALF filter coefficients in the CC-ALF can be constrainedto be less than or equal to K bits such that simpler multipliers (e.g.,having less bits) for the CC-ALF can be used.

In an embodiment, the range of the CC-ALF filter coefficients isconstrained between −2⁴ to 2⁴-1 where K is 5 bits. Alternatively, therange of the CC-ALF filter coefficients is constrained between −2⁵ to2⁵-1 where K is 6 bits.

In an example, a number of different values of the CC-ALF filtercoefficients is constrained to be a certain number, such as K bits. Alookup table can be used when applying the CC-ALF.

The CC-ALF filter coefficients of the CC-ALF can be coded and signaledusing fixed-length coding. For example, if the CC-ALF filtercoefficients are constrained to be K bits, K bits fixed-length codingcan be used to signal the CC-ALF filter coefficients. When K isrelatively small, such as 8 bits, the fixed-length coding can be moreefficient than other methods, such as variable-length coding.

Referring back to FIG. 13, according to aspects of the disclosure, lumasample values (e.g., (1341)) of the luma CB can be shifted to have adynamic range of L bits if the dynamic range (or a luma bit-depth) ofthe luma sample values (e.g., (1341)) is larger than L bits. L can be apositive integer, such as 8. Subsequently, the intermediate component(e.g., (1342) or (1343)) can be generated by applying the CC-ALF (e.g.,(1321) or (1331)) to the shifted luma sample values.

Referring back to FIG. 13, the cross-component adaptive loop filteringprocess using the CC-ALF (e.g., (1321) or (1331)) can be modified asdescribed below if the luma bit-depth of the luma sample values (e.g.,the luma sample values of the SAO filtered luma component (1341)) ishigher than L-bits. The luma sample values (e.g., (1341)) can be firstshifted to an L-bit dynamic range. The shifted luma sample values can beunsigned L-bits. In an example, L is 8. Subsequently, the shifted lumasample values can be used as an input to the CC-ALF (e.g., (1321) or(1331)). Thus, the shifted luma sample values and the CC-ALF filtercoefficients can be multiplied. As described above, the CC-ALF filtercoefficients can be constrained to signed values of less than or equalto K bits (e.g., 8 bits or [−2^(K−1) to 2^(K−1)−1]), a multiplier ofunsigned L-bits by signed K-bits may be used. In an example, K and L are8 bits, and a relatively simple and efficient multiplier (e.g., amultiplier based on single instruction, multiple data (SIMD)instructions) of unsigned 8-bits by signed 8-bits may be used to improvethe filtering efficiency.

Referring to FIG. 13, according to aspects of the disclosure, adown-sampled luma CB can be generated by applying a down-sampling filterto the luma CB. Thus, a chroma horizontal subsampling factor and achroma vertical subsampling factor between the first chroma CB (or thesecond chroma CB) and the down-sampled luma CB is one. The down-samplingfilter can be applied at any suitable step before the down-sampled lumaCB is used as an input to the CC-ALF (e.g., (1321)). In an example, thedown-sampling filter is applied between the SAO filter (1310) and theCC-ALF (e.g., (1321)), and thus the SAO filtered luma component (1341)is down-sampled first and then the down-sampled and SAO filtered lumacomponent is sent to the CC-ALF (1321).

As described above, the filter shape of the CC-ALF can be determinedbased on the chroma subsampling format and/or the chroma sample type,and thus in some examples, different filter shapes can be used fordifferent chroma sample types. Alternatively, the CC-ALF (e.g., (1321))can use a unified filter shape when the input to the CC-ALF is thedown-sampled luma CB as the down-sampled luma samples are aligned withthe chroma samples with the chroma horizontal subsampling factor and thechroma vertical subsampling factor being one. The unified filter shapecan be independent of the chroma subsampling format and the chromasample type of the chroma CB. Accordingly, an intermediate CB (e.g., theintermediate component (1342)) can be generated by applying the CC-ALFhaving the unified filter shape to the down-sampled luma CB.

Referring to FIG. 13, in an example, for the chroma subsampling formatthat is the YUV (e.g., YCbCr or YCgCo) format, a down-sampling filter isapplied to the luma samples in the luma CB to derive down-sampled lumasamples whose positions are aligned with the chroma samples in the firstchroma CB, then a unified filtering shape can be applied in the CC-ALF(e.g., (1321)) to cross-filter the down-sampled luma samples to generatethe intermediate component (1342).

The down-sampling filter can be any suitable filter. In an example, thedown-sampling filter corresponds to a filter applied to co-located lumasamples in a CCLM mode. Thus, the luma samples are down-sampled usingthe same down-sampling filter that is applied to the co-located lumasamples in the CCLM mode.

In an example, the down-sampling filter is a {1,2,1;1,2,1}/8 filter andthe chroma subsampling format is 4:2:0. Thus, for the chroma 4:2:0format, the luma samples are down-sampled by applying the {1,2,1;1,2,1}/8 filter.

The filter shape (or the unified filtering shape) of the cross-componentfilter (e.g., the CC-ALF) can have any suitable shape. In an example,the filter shape of the cross-component filter (e.g., the CC-ALF) is oneof a 7×7 diamond shape, a 7×7 square shape, a 5×5 diamond shape, a 5×5square shape, a 3×3 diamond shape, and a 3×3 square shape.

FIG. 17 shows a flow chart outlining a process (1700) according to anembodiment of the disclosure. The process (1700) can be used toreconstruct a block (e.g., a CB) in a picture of a coded video sequence.The process (1700) can be used in the reconstruction of the block so togenerate a prediction block for the block under reconstruction. The termblock may be interpreted as a prediction block, a CB, a CU, or the like.In various embodiments, the process (1700) are executed by processingcircuitry, such as the processing circuitry in the terminal devices(310), (320), (330) and (340), the processing circuitry that performsfunctions of the video encoder (403), the processing circuitry thatperforms functions of the video decoder (410), the processing circuitrythat performs functions of the video decoder (510), the processingcircuitry that performs functions of the video encoder (603), and thelike. In some embodiments, the process (1700) is implemented in softwareinstructions, thus when the processing circuitry executes the softwareinstructions, the processing circuitry performs the process (1700). Theprocess starts at (S1701) and proceeds to (S1710). In an example, theblock is a chroma block, such as a chroma CB, corresponding to a lumaCB. In an example, the chroma block and the corresponding luma CB is ina CU.

At (S1710), coded information of the chroma CB can be decoded from acoded video bitstream. The coded information can indicate that across-component filter is applied to the chroma CB and can furtherindicate a chroma subsampling format and a chroma sample type. Thechroma subsampling format can indicate a chroma horizontal subsamplingfactor and a chroma vertical subsampling factor between the chroma CBand the corresponding luma CB, as described above. The chromasubsampling format can be any suitable format, such as 4:2:0, 4:2:2,4:4:4, or the like. The chroma sample type can indicate a relativeposition of a chroma sample with respect to at least one correspondingluma sample in the luma CB, as described above. In an example, for thechroma subsampling format of 4:2:0, the chroma sample type can be one ofthe chroma sample types 0-5 described above with reference to FIGS.15A-15B.

At (S1720), a filter shape of the cross-component filter can bedetermined based on at least one of the chroma subsampling format andthe chroma sample type. In an example, referring to FIG. 13, thecross-component filter is used in a cross-component filtering process(e.g., a CC-ALF filtering process) and the cross-component filter can bethe CC-ALF. The filter shape can be any suitable shape that is dependenton the chroma subsampling format and/or the chroma sample type. For thechroma subsampling format of 4:2:0, the filter shape can be one of thefilter shapes (1420) and (1621)-(1623) based on the chroma sample type.The filter shape can be a variation (e.g., a geometric transformationsuch as a rotation or a shift) of one of the filter shapes (1420) and(1621)-(1623) based on the chroma sample type.

At (S1730), a first intermediate CB can be generated by applying a loopfilter (e.g., the ALF) to the chroma CB (e.g., a SAO filtered chromaCB).

At (S1740), a second intermediate CB can be generated by applying thecross-component filter (e.g., the CC-ALF) having the determined filtershape (e.g., the filter shape (1420)) to the corresponding luma CB, forexample, when the chroma subsampling format is 4:2:0 and the chromasample type is the chroma sample 0.

At (S1750), a filtered chroma CB (e.g., the filtered first chromacomponent (1362)) can be determined based on the first intermediate CB(e.g., the intermediate component (1342)) and the second intermediate CB(e.g., the intermediate component (1352)). The process (1700) proceedsto (S1799), and terminates.

The process (1700) can be suitably adapted. Step(s) in the process(1700) can be modified and/or omitted. Additional step(s) can be added.Any suitable order of implementation can be used.

Embodiments in the disclosure may be used separately or combined in anyorder. Further, each of the methods (or embodiments), an encoder, and adecoder may be implemented by processing circuitry (e.g., one or moreprocessors or one or more integrated circuits). In one example, the oneor more processors execute a program that is stored in a non-transitorycomputer-readable medium.

The techniques described above, can be implemented as computer softwareusing computer-readable instructions and physically stored in one ormore computer-readable media. For example, FIG. 18 shows a computersystem (1800) suitable for implementing certain embodiments of thedisclosed subject matter.

The computer software can be coded using any suitable machine code orcomputer language, that may be subject to assembly, compilation,linking, or like mechanisms to create code comprising instructions thatcan be executed directly, or through interpretation, micro-codeexecution, and the like, by one or more computer central processingunits (CPUs), Graphics Processing Units (GPUs), and the like.

The instructions can be executed on various types of computers orcomponents thereof, including, for example, personal computers, tabletcomputers, servers, smartphones, gaming devices, internet of thingsdevices, and the like.

The components shown in FIG. 18 for computer system (1800) are exemplaryin nature and are not intended to suggest any limitation as to the scopeof use or functionality of the computer software implementingembodiments of the present disclosure. Neither should the configurationof components be interpreted as having any dependency or requirementrelating to any one or combination of components illustrated in theexemplary embodiment of a computer system (1800).

Computer system (1800) may include certain human interface inputdevices. Such a human interface input device may be responsive to inputby one or more human users through, for example, tactile input (such as:keystrokes, swipes, data glove movements), audio input (such as: voice,clapping), visual input (such as: gestures), olfactory input (notdepicted). The human interface devices can also be used to capturecertain media not necessarily directly related to conscious input by ahuman, such as audio (such as: speech, music, ambient sound), images(such as: scanned images, photographic images obtain from a still imagecamera), video (such as two-dimensional video, three-dimensional videoincluding stereoscopic video).

Input human interface devices may include one or more of (only one ofeach depicted): keyboard (1801), mouse (1802), trackpad (1803), touchscreen (1810), data-glove (not shown), joystick (1805), microphone(1806), scanner (1807), camera (1808).

Computer system (1800) may also include certain human interface outputdevices. Such human interface output devices may be stimulating thesenses of one or more human users through, for example, tactile output,sound, light, and smell/taste. Such human interface output devices mayinclude tactile output devices (for example tactile feedback by thetouch-screen (1810), data-glove (not shown), or joystick (1805), butthere can also be tactile feedback devices that do not serve as inputdevices), audio output devices (such as: speakers (1809), headphones(not depicted)), visual output devices (such as screens (1810) toinclude CRT screens, LCD screens, plasma screens, OLED screens, eachwith or without touch-screen input capability, each with or withouttactile feedback capability-some of which may be capable to output twodimensional visual output or more than three dimensional output throughmeans such as stereographic output; virtual-reality glasses (notdepicted), holographic displays and smoke tanks (not depicted)), andprinters (not depicted).

Computer system (1800) can also include human accessible storage devicesand their associated media such as optical media including CD/DVD ROM/RW(1820) with CD/DVD or the like media (1821), thumb-drive (1822),removable hard drive or solid state drive (1823), legacy magnetic mediasuch as tape and floppy disc (not depicted), specialized ROM/ASIC/PLDbased devices such as security dongles (not depicted), and the like.

Those skilled in the art should also understand that term “computerreadable media” as used in connection with the presently disclosedsubject matter does not encompass transmission media, carrier waves, orother transitory signals.

Computer system (1800) can also include an interface (1854) to one ormore communication networks (1855). Networks can for example bewireless, wireline, optical. Networks can further be local, wide-area,metropolitan, vehicular and industrial, real-time, delay-tolerant, andso on. Examples of networks include local area networks such asEthernet, wireless LANs, cellular networks to include GSM, 3G, 4G, 5G,LTE and the like, TV wireline or wireless wide area digital networks toinclude cable TV, satellite TV, and terrestrial broadcast TV, vehicularand industrial to include CANBus, and so forth. Certain networkscommonly require external network interface adapters that attached tocertain general purpose data ports or peripheral buses (1849) (such as,for example USB ports of the computer system (1800)); others arecommonly integrated into the core of the computer system (1800) byattachment to a system bus as described below (for example Ethernetinterface into a PC computer system or cellular network interface into asmartphone computer system). Using any of these networks, computersystem (1800) can communicate with other entities. Such communicationcan be uni-directional, receive only (for example, broadcast TV),uni-directional send-only (for example CANbus to certain CANbusdevices), or bi-directional, for example to other computer systems usinglocal or wide area digital networks. Certain protocols and protocolstacks can be used on each of those networks and network interfaces asdescribed above.

Aforementioned human interface devices, human-accessible storagedevices, and network interfaces can be attached to a core (1840) of thecomputer system (1800).

The core (1840) can include one or more Central Processing Units (CPU)(1841), Graphics Processing Units (GPU) (1842), specialized programmableprocessing units in the form of Field Programmable Gate Areas (FPGA)(1843), hardware accelerators for certain tasks (1844), graphics adapter(1850), and so forth. These devices, along with Read-only memory (ROM)(1845), Random-access memory (1846), internal mass storage such asinternal non-user accessible hard drives, SSDs, and the like (1847), maybe connected through a system bus (1848). In some computer systems, thesystem bus (1848) can be accessible in the form of one or more physicalplugs to enable extensions by additional CPUs, GPU, and the like. Theperipheral devices can be attached either directly to the core's systembus (1848), or through a peripheral bus (1849). In an example, a display(1810) can be connected to the graphics adapter (1850). Architecturesfor a peripheral bus include PCI, USB, and the like.

CPUs (1841), GPUs (1842), FPGAs (1843), and accelerators (1844) canexecute certain instructions that, in combination, can make up theaforementioned computer code. That computer code can be stored in ROM(1845) or RAM (1846). Transitional data can be also be stored in RAM(1846), whereas permanent data can be stored for example, in theinternal mass storage (1847). Fast storage and retrieve to any of thememory devices can be enabled through the use of cache memory, that canbe closely associated with one or more CPU (1841), GPU (1842), massstorage (1847), ROM (1845), RAM (1846), and the like.

The computer readable media can have computer code thereon forperforming various computer-implemented operations. The media andcomputer code can be those specially designed and constructed for thepurposes of the present disclosure, or they can be of the kind wellknown and available to those having skill in the computer software arts.

As an example and not by way of limitation, the computer system havingarchitecture (1800), and specifically the core (1840) can providefunctionality as a result of processor(s) (including CPUs, GPUs, FPGA,accelerators, and the like) executing software embodied in one or moretangible, computer-readable media. Such computer-readable media can bemedia associated with user-accessible mass storage as introduced above,as well as certain storage of the core (1840) that are of non-transitorynature, such as core-internal mass storage (1847) or ROM (1845). Thesoftware implementing various embodiments of the present disclosure canbe stored in such devices and executed by core (1840). Acomputer-readable medium can include one or more memory devices orchips, according to particular needs. The software can cause the core(1840) and specifically the processors therein (including CPU, GPU,FPGA, and the like) to execute particular processes or particular partsof particular processes described herein, including defining datastructures stored in RAM (1846) and modifying such data structuresaccording to the processes defined by the software. In addition or as analternative, the computer system can provide functionality as a resultof logic hardwired or otherwise embodied in a circuit (for example:accelerator (1844)), which can operate in place of or together withsoftware to execute particular processes or particular parts ofparticular processes described herein. Reference to software canencompass logic, and vice versa, where appropriate. Reference to acomputer-readable media can encompass a circuit (such as an integratedcircuit (IC)) storing software for execution, a circuit embodying logicfor execution, or both, where appropriate. The present disclosureencompasses any suitable combination of hardware and software.

APPENDIX A: ACRONYMS

-   JEM: joint exploration model-   VVC: versatile video coding-   BMS: benchmark set-   MV: Motion Vector-   HEVC: High Efficiency Video Coding-   MPM: most probable mode-   WAIP: Wide-Angle Intra Prediction-   SEI: Supplementary Enhancement Information-   VUI: Video Usability Information-   GOPs: Groups of Pictures-   TUs: Transform Units,-   PUs: Prediction Units-   CTUs: Coding Tree Units-   CTBs: Coding Tree Blocks-   PBs: Prediction Blocks-   HRD: Hypothetical Reference Decoder-   SDR: standard dynamic range-   SNR: Signal Noise Ratio-   CPUs: Central Processing Units-   GPUs: Graphics Processing Units-   CRT: Cathode Ray Tube-   LCD: Liquid-Crystal Display-   OLED: Organic Light-Emitting Diode-   CD: Compact Disc-   DVD: Digital Video Disc-   ROM: Read-Only Memory-   RAM: Random Access Memory-   ASIC: Application-Specific Integrated Circuit-   PLD: Programmable Logic Device-   LAN: Local Area Network-   GSM: Global System for Mobile communications-   LTE: Long-Term Evolution-   CANBus: Controller Area Network Bus-   USB: Universal Serial Bus-   PCI: Peripheral Component Interconnect-   FPGA: Field Programmable Gate Areas-   SSD: solid-state drive-   IC: Integrated Circuit-   CU: Coding Unit-   PDPC: Position Dependent Prediction Combination-   ISP: Intra Sub-Partitions-   SPS: Sequence Parameter Setting

While this disclosure has described several exemplary embodiments, thereare alterations, permutations, and various substitute equivalents, whichfall within the scope of the disclosure. It will thus be appreciatedthat those skilled in the art will be able to devise numerous systemsand methods which, although not explicitly shown or described herein,embody the principles of the disclosure and are thus within the spiritand scope thereof.

What is claimed is:
 1. A method for video encoding in an encoder,comprising: determining a filter shape of a cross-component filterapplied to a chroma coding block (CB); generating a first intermediateCB by applying a loop filter to the chroma CB; generating a secondintermediate CB by applying, to a corresponding luma CB, thecross-component filter applied to the chroma CB and having thedetermined filter shape; determining a filtered chroma CB based on thefirst intermediate CB and the second intermediate CB by combining theloop filtered chroma CB with the cross-component filtered luma CB; andgenerating coded information of the chroma CB in a coded videobitstream, the coded information including (i) an indication that thecross-component filter is applied to the chroma CB, (ii) an indicationof a chroma subsampling format and a chroma sample type that indicates arelative position of a chroma sample with respect to at least one lumasample in the corresponding luma CB, and (iii) a number of filtercoefficients of the cross-component filter, wherein the determining thefilter shape includes determining the filter shape of thecross-component filter based on the number of the filter coefficientsand based on at least one of (i) the chroma subsampling format or (ii)the chroma sample type.
 2. The method of claim 1, wherein the generatedcoded information in the coded video bitstream signals the chroma sampletype.
 3. The method of claim 1, wherein the chroma subsampling format is4:2:0; the at least one luma sample includes four luma samples that area top-left sample, a top-right sample, a bottom-left sample, and abottom-right sample; the chroma sample type is one of six chroma sampletypes 0-5 indicating six relative positions 0-5, respectively, and thesix relative positions 0-5 of the chroma sample correspond to aleft-center position between the top-left and the bottom-left samples, acenter position of the four luma samples, a top left position co-locatedwith the top-left sample, a top-center position between the top-left andthe top-right samples, a bottom left position co-located with thebottom-left sample, and a bottom-center position between the bottom-leftand the bottom-right samples, respectively; and the determining thefilter shape includes determining the filter shape of thecross-component filter based on the chroma sample type.
 4. The method ofclaim 3, wherein the coded video bitstream includes a cross-componentlinear model (CCLM) flag indicating that the chroma sample type is 0 or2.
 5. The method of claim 1, wherein the cross-component filter is across-component adaptive loop filter (CC-ALF) and the loop filter is anadaptive loop filter (ALF).
 6. The method of claim 1, wherein a range ofthe filter coefficients of the cross-component filter is less than orequal to K bits and K is a positive integer.
 7. The method of claim 6,wherein the filter coefficients of the cross-component filter are codedand signaled using fixed-length coding.
 8. The method of claim 6,further comprising: shifting luma sample values of the correspondingluma CB to have a dynamic range of 8 bits based on the dynamic range ofthe luma sample values being larger than 8 bits, K being 8 bits, whereinthe generating the second intermediate CB includes applying thecross-component filter having the determined filter shape to the shiftedluma sample values.
 9. A method for video encoding in an encoder,comprising: generating a down-sampled luma coding block (CB) by applyinga down-sampling filter to a luma CB corresponding to a chroma CB, achroma horizontal subsampling factor, and a chroma vertical subsamplingfactor between the chroma CB and the down-sampled luma CB being one;generating a first intermediate CB by applying a loop filter to thechroma CB; generating a second intermediate CB by applying, to thedown-sampled luma CB, a cross-component filter applied to the chroma CB,a filter shape of the cross-component filter being independent of achroma subsampling format and a chroma sample type of the chroma CB, thechroma sample type indicating a relative position of a chroma samplewith respect to at least one luma sample in the corresponding luma CB;determining a filtered chroma CB based on the first intermediate CB andthe second intermediate CB by combining the loop filtered chroma CB withthe cross-component filtered down-sampled luma CB; and generating codedinformation of the chroma CB in a coded video bitstream, the codedinformation including (i) an indication that the cross-component filteris applied to the chroma CB based on a corresponding luma CB, and (ii) anumber of filter coefficients of the cross-component filter, wherein thefilter shape of the cross-component filter is determined based on thenumber of filter coefficients.
 10. The method of claim 9, wherein thedown-sampling filter corresponds to a filter applied to co-located lumasamples in a cross-component linear model (CCLM) mode.
 11. The method ofclaim 9, wherein the down-sampling filter is a {1,2,1;1,2,1}/8 filterand the chroma subsampling format is 4:2:0.
 12. The method of claim 9,wherein the filter shape of the cross-component filter is one of a 7×7diamond shape, a 7×7 square shape, a 5×5 diamond shape, a 5×5 squareshape, a 3×3 diamond shape, and a 3×3 square shape.
 13. An apparatus forvideo encoding, comprising: processing circuitry configured to:determine a filter shape of a cross-component filter applied to a chromacoding block (CB); generate a first intermediate CB by applying a loopfilter to the chroma CB; generate a second intermediate CB by applying,to a corresponding luma CB, the cross-component filter applied to thechroma CB and having the determined filter shape; determine a filteredchroma CB based on the first intermediate CB and the second intermediateCB by combining the loop filtered chroma CB with the cross-componentfiltered luma CB; and generate coded information of the chroma CB in acoded video bitstream, the coded information including (i) an indicationthat the cross-component filter is applied to the chroma CB, (ii) anindication of a chroma subsampling format and a chroma sample type thatindicates a relative position of a chroma sample with respect to atleast one luma sample in the corresponding luma CB, and (iii) a numberof filter coefficients of the cross-component filter, wherein the filtershape of the cross-component filter is determined based on the number offilter coefficients and based on at least one of (i) the chromasubsampling format or (ii) the chroma sample type.
 14. The apparatus ofclaim 13, wherein the chroma sample type is signaled in the coded videobitstream.
 15. The apparatus of claim 13, wherein the chroma subsamplingformat is 4:2:0; the at least one luma sample includes four luma samplesthat are a top-left sample, a top-right sample, a bottom-left sample,and a bottom-right sample; the chroma sample type is one of six chromasample types 0-5 indicating six relative positions 0-5, respectively,and the six relative positions 0-5 of the chroma sample correspond to aleft-center position between the top-left and the bottom-left samples, acenter position of the four luma samples, a top left position co-locatedwith the top-left sample, a top-center position between the top-left andthe top-right samples, a bottom left position co-located with thebottom-left sample, and a bottom-center position between the bottom-leftand the bottom-right samples, respectively; and the processing circuitryis configured to determine the filter shape of the cross-componentfilter based on the chroma sample type.
 16. The apparatus of claim 15,wherein the coded video bitstream includes a cross-component linearmodel (CCLM) flag indicating that the chroma sample type is 0 or
 2. 17.The apparatus of claim 13, wherein the cross-component filter is across-component adaptive loop filter (CC-ALF) and the loop filter is anadaptive loop filter (ALF).
 18. The apparatus of claim 13, wherein arange of filter coefficients of the cross-component filter is less thanor equal to K bits and K is a positive integer.
 19. The apparatus ofclaim 18, wherein the filter coefficients of the cross-component filterare coded and signaled using fixed-length coding.
 20. The apparatus ofclaim 18, wherein the processing circuitry is further configured to:shift luma sample values of the corresponding luma CB to have a dynamicrange of 8 bits based on the dynamic range of the luma sample valuesbeing larger than 8 bits, K being 8 bits, wherein the processingcircuitry is configured to generate the second intermediate CB byapplying the cross-component filter having the determined filter shapeto the shifted luma sample values.